Boat Repair 101 - How To Fix Fiberglass Boats Complete Guide

Epoxy Safety 

Precautions 

1. Avoid contact with resin, hardeners, epoxy and sanding dust when possible. Never use any type of solvents to remove epoxy from your skin. Wear all PPE gear, including gloves, respirator and clothing covering your skin whenever you are working near epoxies. If you get resin, hardener or mixed epoxy on your skin, remove it as soon as you can. Resin is not water soluble so your best bet is to use a waterless skin cleanser.  Hardener and dust particles are water soluble so you’re able to wash with soap and water. Always wash thoroughly with soap and warm water after working with any epoxy. 

1. If contact occurs with your eyes, immediately flush the eyes with water under low pressure for 15 minutes. Seek medical attention or advice. 
2. Use adequate ventilation to avoid inhaling vapors, fumes and sanding dust, particularly when working in boat interiors. Epoxies have a low volatile organic content (VOC), but vapors can build up in unventilated spaces. Use a P100 particulate filter when working without adequate ventilation.  Breathing dust particles from uncured epoxy increases your risk of sensitization. Although epoxy cures quickly to a solid that can be sanded, it may take over two weeks at room temperature to cure completely. 
3. Wash thoroughly after handling epoxy, especially before eating or smoking. If epoxy is swallowed, rinse mouth with water & DO NOT induce vomiting. Because hardeners are corrosive, they can cause additional harm if vomited.

Caution! Large pots of curing epoxy can get hot enough to ignite surrounding combustible materials and give off hazardous fumes.

Hazards 

The primary hazard with epoxy involves skin contact. Resins may cause moderate skin irritation. Hardeners are corrosive and may cause severe skin irritation. Resins and hardeners are also sensitizers and may cause an allergic reaction similar to poison ivy. Most people are not sensitive to Resins and Hardeners but the risk of becoming sensitized increases with repeated contact. These hazards also apply to the sanding dust from epoxy that has not fully cured. These hazards decrease as resin/hardener mixtures reach full cure. 

Cleanup 

DO NOT clean up or dispose of hardener in trash containing sawdust as spontaneous combustion can occur. 

Clean resin or mixed epoxy residue with lacquer thinner, acetone or alcohol. 

Clean hardener residue with warm soapy water. 

Dispose of resin, hardener and empty containers safely. DO NOT dispose of resin or hardener in a liquid state. Waste resin and hardener can be mixed and cured (in small quantities) to a non-hazardous inert solid. 

Removing uncured or non-curing epoxy. 

Uncured epoxy is removed as you would spilled resin. Scrape as much material as you can from the surface using a stiff metal or plastic scraper—warm the epoxy to lower its viscosity. Clean the residue with lacquer thinner, acetone, or alcohol.  Allow solvents to dry before coating again. 

Removing fiberglass cloth applied with epoxy. 

Use a heat gun to soften the epoxy. Start in a small area near an edge. Apply heat until you can slip a putty knife under the cloth (about 250°F). Grab the edge with a pair of pliers and slowly pull up on the cloth while heating just ahead of the separation. On large areas, use a utility knife to score the glass and remove in narrower strips. Resulting surface texture may be coated or remaining epoxy may be removed as follows. Provide ventilation or wear a respirator when heating epoxy. 

Removing cured epoxy coating. 

Use a heat gun to soften the epoxy (about 250°F). Heat a small area and use a paint or cabinet scraper to remove the bulk of the coating. Sand the surface to remove the remaining material. Provide ventilation or wear a respirator when heating epoxy. 

Epoxy Chemistry 

Epoxy’s Cure Stages 

Mixing Epoxy Resin with a hardener begins a chemical reaction that transforms the combined liquid ingredients to a solid. This period is the cure time. As it cures, epoxy passes from the liquid state, through a gel state, before it reaches a solid state.

Cure time is shorter when the epoxy is warmer. 

Cure time is longer when the epoxy is cooler. 

Liquid—Open Time

Open time (also working time or wet lay-up time) is the portion of the cure time, after mixing, that the resin/hardener mixture remains a liquid and is workable and suitable for application. All assembly and clamping should take place during the open time to ensure a dependable bond. 

Gel—Initial Cure Phase 

The mixture passes into an initial cure phase (also called the green stage) when it begins to gel, or “kick-off.” The epoxy is no longer workable and will progress from a tacky, gel consistency to the firmness of hard rubber, which you will be able to dent with your thumbnail. The mixture will become tack free about midway through the initial cure phase. While it is still tacky (about like masking tape), a new application of epoxy will still chemically bond with it, so you may still bond to or coat the surface again without special preparation. However, this ability diminishes as the mixture approaches the final cure phase. 

Solid—Final Cure Phase 

The epoxy mixture has cured to a solid state and can be dry sanded. You will no longer be able to dent it with your thumbnail. At this point the epoxy has reached most of its ultimate strength, so clamps can be removed. A new application of epoxy will no longer chemically bond to it, so the surface of the epoxy must be properly prepared and sanded before re-coating to achieve a good mechanical, secondary bond. 

Surface Preparation

CAUTION! Heating epoxy that has not gelled will lower its viscosity, allowing the epoxy to run or sag more easily on vertical surfaces. In addition, heating epoxy applied to a porous substrate (soft wood or low density core material) may cause the substrate to “out-gas” and form bubbles or pinholes in the epoxy coating. To avoid out-gassing, wait until the epoxy coating has gelled before warming it. Never heat mixed epoxy in a liquid state over 120°F (49°C). Regardless of what steps are taken to control the cure time, thorough planning of the application and assembly will allow you to make maximum use of epoxy’s open time and cure time. 

You can improve epoxy’s thermal performance and reduce the potential for fabric “print-through” by applying modest heat to the epoxy after it has cured to a solid state. 

WARNING! Curing epoxy generates heat. Do not fill voids or cast layers of epoxy thicker than 1⁄2" thinner if enclosed by foam or other insulating material. Several inches of mixed epoxy in a confined mass (such as a mixing cup) will generate enough heat to melt a plastic cup, burn your skin or ignite combustible materials if left to stand for its full pot life. 

Controlling Cure Time 

Open time and overall cure time distinguish much of the activity of building and repairing with epoxy. Open time is the time available for mixing, application, smoothing, shaping, assembly and clamping. Cure time is how long you must wait before removing clamps, or before you can sand or go on to the next step in the project. Two factors determine an epoxy mixture’s open time and overall cure time—hardener and epoxy temperature. 

1. Hardener Cure Speed 

Each hardener has an ideal temperature cure range. At any given temperature, each resin/hardener combination will go through the same cure stages, but at different time rates. Select the hardener that gives you enough working time for the job you are doing at the temperature and conditions you are working with. 

Pot life is a term used to compare the cure speeds of different hardeners. It is the amount of time a specific mass of mixed resin and hardener remains a liquid at a specific temperature.  Pot life is a measure of the cure speed of a specific contained mass (volume) of epoxy rather than a thin film, a hardener’s pot life is much shorter than its open time. 

2. Epoxy Temperature 

The hot ethe temperature of curing epoxy, the faster it cures. The temp. of curing epoxy is determined by the ambient temperature plus the exothermic heat generated by its cure. 

Exothermic heat is produced by the chemical reaction that cures epoxy. The amount
of heat produced depends on the thickness or exposed surface area of mixed epoxy.
When thicker, more heat is retained, causing a faster reaction and more heat. The mixing container’s shape and the mixed quantity have a great effect on this exothermic reaction. An 8 fl. oz. or more scoop of epoxy in a plastic mixing cup can quickly generate enough heat to melt the cup and burn your skin. But if the same amount is spread into a thin layer, exothermic heat is released, and the epoxy’s cure time is determined by the ambient temperature. The thinner the layer of curing epoxy, the less it is affected by exothermic heat, and the slower it cures.

Adapting to Warm and Cool Weather 

In warm weather, gain open time by using a slower hardener. Mix small batches that can be used quickly, or pour the epoxy mixture into a container with greater surface area (a roller pan, for example),  allowing exothermic heat to dissipate and extend the open time. The sooner the mixture is transferred or applied (after mixing), the more of the mixture’s useful open time will be available for coating, lay-up and assembly. 

In cool weather, use a fast hardener, or use electric heat to adjust the epoxy temperature above the hardener’s  recommended application temperature. Use a hot air gun, heat lamp or other heat source to warm the resin and hardener before mixing or after the epoxy is applied. At room temperature, supplemental heat is useful when a quicker cure is desired.

Beware! Unvented kerosene or propane heaters can inhibit the cure of epoxy and contaminate epoxy surfaces with unburned hydrocarbons. 

Dispensing and Resin and Hardeners 

Accurate measuring of epoxy resin and hardener along with a thorough mixing are essential for a proper cure. Whether the resin/hardener mixture is used as a coating or modified with fillers or additives, taking note to the following procedures will make sure of a controlled and thorough chemical transition to a high-strength epoxy solid. 

Dispensing 

Dispense the correct amount of resin and hardener into a clean plastic, metal or wax-free paper container. Do not use glass or foam containers because of the potential danger from exothermic heat buildup within itself. 

DO NOT change the epoxy cure time by adjusting the mix ratio. An accurate ratio is needed for the epoxy to properly cure.

First Time Users 

If this is the first time you have used Epoxy, start with a small batch to get the idea for the mixing and curing process before starting on your project. This will show the hardener’s open time for the temperature you are working in and let you know that the resin/hardener ratio is mixed properly. Mix small batches until you become more confident of the mixture’s handling characteristics. 

Mixing 

Mix the two items together thoroughly, for at least 1 minute, longer in cooler temperatures.  Make sure to thoroughly mix, scrape the sides and bottom of the cup as you continue to mix. Use the flat end of the mixing stick to reach the inside corner of the cup.  If you are going to be using the mixture for coating, quickly pour it into a roller pan to extend the life before it cures. 

Fillers 

After selecting an appropriate filler for the job, use it to thicken the epoxy mixture to the desired consistency. The thickness of a mixture required is controlled by the amount of filler being added. There is no strict formula or measuring involved—use your best judgement with what consistency works best each time. 

Always Add Fillers in a 2 step Process: 

1.. Mix the desired quantity of resin and hardener thoroughly before adding fillers. Begin with a small batch—leave room for the filler in cup. 

2. Blend in small amounts of the appropriate filler until the desired consistency is achieved. 

For maximum strength, add only as much filler to bridge gaps between surfaces without sagging or running out of the joint or gap. A small amount of epoxy should squeeze out of joints when clamped together. For thicker mixtures, don’t fill the cup with more than 1/3 of epoxy before adding filler. When making fairing compounds, stir in as much filler as you can blend in smoothly—for easy sanding, the thicker the better. Be sure all of the filler is completely blended together before the mixture is applied. 

Thinning Epoxy 

Now there are epoxy-based products designed specifically to penetrate and reinforce rotted wood. Those products, an epoxy thinned with solvents, do a good job of penetrating wood. Tthe solvents compromise the strength and moisture barrier properties of the epoxy. Epoxy can be thinned with solvents for better penetration, but will result in  compromises in strength and moisture resistance. Acetone and lacquer thinner can been used to thin Epoxy and duplicate these penetrating epoxies. Keep in mind that the strength, especially compressive strength, and moisture protection of the epoxy is lost in proportion to the amount of solvent added. 

Its best to heat the project area up to 120°F with a heat gun or heat lamp before applying epoxy to get better penetration without losing strength or moisture resistance. On contact with the warmed fiberglass, the epoxy will thin out, penetrating cavities and pores, and will be sucked even deeper into pores as the substrate begins to cool off.  The working time of the epoxy will be considerably shortened, slower hardeners will have a much longer working life and will penetrate more than Hardener before they turn to gel. When the epoxy fully cures it will retain all of its strength and effectiveness as a moisture barrier.

Additives 

Additives are used to give epoxy additional physical properties when used as an additional coating. Although additives are blended together with mixed epoxy in the same two-step process as fillers, they are not designed to thicken the epoxy. 

Coloring Epoxy 

Pigments are available to dye epoxy black, white or gray. Powdered pigments and universal tinting pigment can be added to the epoxy mixture to tint any color. Acrylic paste pigments can  be used to tint the mixture, as long as they are specified for use with polyester or epoxy resin. Graphite Powder will also change the color of the epoxy black or impart darker shades to colors. 

Coloring agents can be added to the mixed epoxy up to 5% by volume with minimal little to no effect on the cured epoxy’s strength. Try with test samples to check for desired color and opaqueness and for proper cure. None of these coloring additives provide UV resistance to the cured epoxy, so limit their use to areas not exposed to sunlight unless additional UV protection is applied. 

Cold Weather

Epoxy can be used under cold weather conditions, but you must use special application techniques. These precautions are not elaborate or difficult, but they are necessary to achieve acceptable long-term epoxy performance. These precautions do not apply to Epoxy alone; any epoxy used in critical marine structural situations may have its capabilities and performance affected by cold weather. In fact, due to differences in formulation, not all epoxies possess the necessary characteristics to ever cure well under cold weather conditions. 

Chemical characteristics 

When you mix an epoxy resin and hardener together, you start a chemical reaction which, as a byproduct, produces heat. This is called an exothermic reaction. The ambient temperature in which an epoxy chemical reaction takes place affects the rate of reaction. Warmer temperatures accelerate the reaction time, while cooler temperatures retard it. 

Duration of reaction, among other variables, influences inter-bonding of the epoxy molecules. If the reaction is too slow, even though the epoxy may harden, it may not cure completely and possibly never achieve its designed physical properties. This is where danger lies, for improperly cured epoxy may possess enough strength to hold a structure together, yet it may fail after repeated loadings during normal operation. 

Working properties 

Temperature has a profound effect on the working properties of uncured epoxy. Ambient temperature changes will drastically change the epoxy’s viscosity (thickness). Viscosity of water varies little with temperature changes until it either boils or freezes. Epoxy, however, is made of heavier molecules and temperature can have a 10 times greater effect on epoxy molecules than on water molecules over a temperature change of 30°F (16°C).

The colder it gets, the thicker the epoxy becomes, reducing its ability to flow out. This kind of change has three important consequences for working with epoxy under cold conditions. 

First, it is more difficult to mix the resin and hardener thoroughly: the resin flows through the dispensing pumps and out of containers with greater difficulty; the cold resin and hardener are prone to clinging to the surfaces of the pumps, containers and mixing tools; and they resist being completely blended unless mixed very thoroughly. Remember, because of the low temperature, the chemical reaction isn’t going off as well either. Compounding a less efficient exothermic reaction with potential for incomplete and/or inaccurate mixing, you have the recipe for a permanently deficient bond. 

Second, the mixed epoxy is much harder to apply. If you’ve ever tried to spoon honey right out of the refrigerator instead of at room temperature, you know just what we’re referring to: the chilled mixture has become stiff. When cold temperatures make epoxy stiff, it’s extremely difficult to coat and wet out surfaces. 

Third, air bubbles may be introduced when mixing and held in suspension due to the chilled epoxy’s increased surface tension. This can be especially troublesome in clear-finish applications. 

Cold weather techniques 

Use Fast Hardener 

Epoxy Hardener has been designed with a chemically-activated polyamine which exhibits a good cure as low as 35°F (1.5°C). It exhibits a faster cure characteristic than Slow Hardener and offers less uncured exposure time which reduces the chances of incomplete cure due to the cold temperature. 

Dispense resin and hardener in the proper mixing ratio 

All epoxies have been formulated for a specific mixing ratio of resin to hardener. It is important to mix your epoxy in the precise ratio recommended by the manufacturer. Increasing the amount of hardener will not accelerate cure, but it will seriously compromise the epoxy’s ultimate strength. Epoxy Mini Pumps are designed and calibrated to dispense the correct ratio with one pump stroke of resin for every one pump stroke of hardener. 

Warm resin and hardener before using 

As we discussed above, the warmer the resin and hardener, the lower the viscosity. Thinner resin and hardener will flow through mechanical pumps better, cling less to containers and mixing equipment, and exhibit superior handling and wet-out characteristics. The epoxy can be warmed using heat lamps or can simply be kept in a warm area until you are ready to use it. Another simple method of warming the resin and hardener is to construct a small hot box out of rigid sheets of foil-backed insulation. Place a regular light bulb or an electric heating pad inside to maintain a temperature of no greater than 90°F (32°C). 

Stir the resin and hardener thoroughly

Use extra care when mixing the resin and hardener, and mix for longer than normal periods of time. Scrape the sides and bottom of the mixing container, using a flat-ended mixing stick to reach the corners. Using a smaller diameter mixing pot will also improve the chemical activity because the limited surface area will not dissipate heat produced by the reaction. 

Warm working surfaces 

Applying warmed epoxy to a cold structure will quickly retard the molecular bonding activity of the epoxy. Be certain the structure, as well as the area surrounding the structure, is brought up to temperature. A hull, for example, which is colder than the surrounding air may experience condensation and result in water contamination to epoxy applied on it. Warm the structure as much as possible. This can be done by constructing tents around small areas and heating with portable heaters or warming the area with hot air guns or heat lamps. Small components or materials (such as fiberglass cloth) can be warmed before use in a hot box as described above. 

Prepare surfaces carefully between applications 

When coating under cold conditions, a thin film of epoxy often dissipates any exothermic heat generated by the reaction. When heat is dissipated quickly, the epoxy may not cure for an extended period of time. Some reaction with water may occur, resulting in the formation of an amine blush on the surface. Immediately prior to applying subsequent coatings, wash the surface with clean water and allow it to dry thoroughly. 

Cold weather storage 

It is best to store epoxy materials above 35°F (1.5°C) with the container caps screwed down tightly. Storing epoxy in extreme cold may cause crystallization. The formation of crystals does not compromise the epoxy and can be remedied. Boil water in a pot large enough to hold your epoxy containers. Remove each container’s lid to avoid pressure buildup, which may cause the cans to burst, and place the cans in the hot water. Continually stir the epoxy with a clean stick until the liquid regains clarity and all crystals have melted. Remove from the water, replace the lids tightly and invert the container to melt any crystals which may be clinging to the top of the container. If the resin pump has crystallized, pumping warm resin through it should dissolve the crystals.

The Problem of Gelcoat Blisters in Fiberglass Boats 

Studies suggest that one in four boats can be expected to blister in its lifetime. As more
is known about blisters and their underlying causes, it is apparent that the problem has become more widespread and runs deeper than just the outward appearance of gelcoat blisters. The extent of blister damage varies from boat to boat. It may appear as a few large isolated blisters or as thousands of small blisters covering an entire hull. In some cases, owners may be unaware that their boats are blistering or that there may be a serious destructive process taking place beneath the gelcoat. 

A fiberglass boat is a laminated structure consisting of layers of various reinforcing fabrics and core materials, typically bonded together with polyester resin, and covered with an outer layer of polyester gelcoat. Blisters occur when water that has penetrated the laminate dissolves water soluble materials within the laminate and accumulates in voids or cavities below the gelcoat layer. The solution of water and water soluble materials, through the process of osmosis, attracts more water to the cavities diluting the solution. The pressure of the accumulating water enlarges the cavities to form gelcoat blisters. 

When water soluble materials in a polyester resin laminate mix with moisture that has penetrated the laminate, it creates an acidic fluid. This acidic mixture can attack the polyester resin throughout the laminate, severing the chemical bonds that hold the resin matrix together, as well as the resin-to-fiber bonds. This process is known as hydrolysis. Once hydrolysis has started in a polyester hull, the hull’s strength has been compromised and the potential for serious additional hydrolysis will never go away. If you own a fiberglass boat built with polyester resin, you should be aware that the potential for this problem exists, and is greater in warmer climates. The extent of damage due to hydrolysis should be included in an assessment of a boat’s condition before repairs are made. Gelcoat blisters can often be repaired before the laminate is damaged by hydrolysis. Keep in mind that gelcoat blisters can be an indicator of hydrolysis, and that hydrolysis can occur without the appearance of blisters. 

Hydrolyzed laminate and gelcoat blisters can be treated with Epoxy to limit further damage and in many cases restore a hull’s structural integrity. In this section we will explain the factors affecting blister formation, describe techniques for repairing blister damage and applying an epoxy barrier coating, and offer steps to help prevent future problems. Our recommendations are based on our experiences coupled with extensive laboratory and field testing since we began the formulation of quality marine epoxies in 1971. 

Although Epoxy has been used to successfully repair and protect thousands of blistered boats, it is just one of the factors contributing to a lasting repair. Thorough preparation, drying, repairing and coating, as well as preventative maintenance after the repair, are also essential for a successful repair–but do not guarantee it. Other factors beyond your control, such as the amount of water soluble materials in the laminate, the level of existing damage, and the boat’s environment also affect the long term success of a blister repair. 

Factors affecting blister formation 

As a building material, the unsaturated polyesters used in fiberglass construction seem to be a logical choice. They offer relative ease of working with, reasonable cost and an acceptable working lifetime. Unfortunately, there are other important characteristics that we now know are working against the polyester structures which lead to problems like blistering and delamination. Many variables affect the formation of blisters including the formulation of the resin for specific applications, manufacturing quality assurance and the boat’s environment. The chemical stability and moisture permeability of the polymer resin matrix are the key items affecting the durability of the fiberglass hull. 

Permeability 

The term permeability refers to the ability of a material to permit a substance to pass through it. Polyester laminating resins and gelcoats are not waterproof; they are permeable and will allow water to migrate through the cured resins at a consistent, predictable rate. The permeability of a polymer matrix involves a number of factors. 

The thickness of the gelcoat layer, the amount of air or voids in the laminate and the temperature of the laminate affect how much water can permeate the laminate. The warmer the ambient temperature, the higher the rate of permeation. An increase in temperature will boost the rate of permeation through a resin matrix by intensifying the molecular motion of both the polyester and the water. This means boats in the Caribbean are more likely to have problems than those in Lake Superior. 

The Problem of Gelcoat Blisters in Fiberglass Boats 

An important factor affecting blister formation is the distribution of free volume (voids) in the matrix. In any laminate, the free volume can be everything from the gaps between and within the molecules of the polymer matrix to manufacturing defects such as entrapped air bubbles, cracks or dry fabric. The cure rate, degree of cross-linking and crystallization all affect this void distribution while also contributing to the overall chemical stability of the cured polymer matrix. When water accumulates in these voids, the formation of blisters is initiated. The acidic fluid that develops may eventually begin to hydrolyze the surrounding resin. Due to manufacturing practices, voids commonly occur at the interface between the gelcoat and the laminate, explaining why a large percentage of blisters develop in this area. 

Osmosis and Hydrolysis 

Water-soluble materials, or solutes (excess glycols, acids, metal salts, etc.), trapped in voids between the laminate and the gelcoat, or within the laminate, are primarily large molecules. The laminate is surrounded by relatively pure small water molecules. The small water molecules move from an area of greater concentration to an area of lesser concentration (the voids), and dissolve the solutes to form a blister fluid solution. The gelcoat and laminate act as a semi-permeable membrane. They allow the small water molecules in, but do not let the larger solution molecules out. 

The Problem of Gelcoat Blisters in Fiberglass Boats 

Polyester resins and gelcoats allow water molecules to migrate into the laminate and dissolve soluble materials within the laminate. This one-way movement of water into the laminate is known as osmosis. More water is attracted to the voids to dilute the concentration of solutes in the solution. The blister fluid continues to accumulate and eventually creates enough hydraulic pressure to result in a blister. 

Under the right conditions, a polymer matrix of polyester resin may degrade when exposed to water. Water in contact with the laminates unreacted resin components (glycols, organic acids, catalyst and metallic accelerator) forms an acid that can breakdown susceptible ester linkages which compromise the majority of bonds in polyester polymers. This chemical reaction is called hydrolysis. Water passing into voids and resin-starved pockets within the laminate helps break down more of the polyester molecular chains, which in turn allows more water to pass into the laminate. The process feeds on itself, creating more blisters and damaging more resin by hydrolysis. Keep in mind that standing water and high humidity in the bilge (due to poor ventilation), also permeate the laminate and contribute to blister formation. 

Formulation variables influencing blister formation 

A large number of formulation variables influence the susceptibility or resistance of cured polyester laminates to degradation and blistering. Many different types and combinations of glycols, acids and reactive diluents can be used by the resin manufacturer when developing a formulation. Each ingredient alters the basic physical characteristics of the cured resin, including hydrolytic stability, strength and elongation. The mixing process can also have an impact if it leaves improperly mixed and unreacted glycols trapped in the resin after cure. 

Particular unsaturated polyester resins, accelerators and catalysts can act as blister initiators in poorly mixed or incompletely reacted matrices. Theoretically, a wide variety of additives (air-release agents, leveling additives, UV-resistant additives, surfactants, abrasion-resistant additives, fire retardants, antioxidants and co-monomers) have the potential to affect blister resistance in the cured laminate. Thixotropic agents, hydrophilic fillers, pigments, color paste vehicles, and the use of solvents as diluents can change the sensitivity to moisture and aid in the formation of blisters. The inclusion of any moisture- sensitive materials could stimulate hydrolysis of the matrix materials and promote the osmotic pressure which causes blisters. 

Post Construction Factors 

Poor quality manufacturing practices, material limitations and the rigors of the boating environment can have an adverse effect on the interfacial adhesion between the polyester and fiber reinforcement in the laminate. In addition to poor wet-out during fabrication, high stress or strain in the laminate during use can cause a loss of adhesion or initiate micro-cracking at the interface. Micro-bubbles and multi-phase interfaces within the matrix (due to different cure and shrinkage rates) are all points of stress concentration and are areas vulnerable to loss of adhesion or cohesion. The resulting voids promote water migration, leading to hydrolysis and the concentration of any soluble materials in the laminate. 

Moisture within the laminate is generally accepted as the common denominator in the gelcoat blister equation. It stands to reason that sound measures toward combating osmotic blistering problems would be drying the laminate thoroughly, keeping the interior of the boat as dry as possible and preventing water from passing through the outside of the hull by providing a water-resistant barrier coating. 

There are three compelling reasons to use an epoxy resin rather than a polyester resin or other materials to combat gelcoat blistering. Epoxy is more effective as a moisture barrier, has greater resistance to hydrolysis and is a better structural adhesive. 

Choosing an Effective Moisture Barrier 

Various common finishes have significantly different levels of moisture exclusion effectiveness. At the end of six weeks of exposure to 90% relative humidity at 80°F (27°C), polyester resin is functioning at 30% MEE. Polyurethane paints, on the other hand, are at nearly 40% MEE with one popular brand and 65% MEE with another. 

It is also significant to note that the MEE slope for Epoxy is relatively flat at six weeks, while the low-performance epoxies, polyurethane paints and polyester resins are suffering from sharply declining curves which indicate that the MEE of these coatings will continue to deteriorate at a rapid pace. 

This research has shown that Epoxy has a much higher resistance to moisture than most other coatings. This is a critical characteristic in reducing moisture permeability through the resin matrix which could result in gelcoat blistering and/or interlaminate failure. 

Hydrolysis

An epoxy matrix is more resistant to hydrolysis than a polyester matrix. The structure of the cured epoxy’s ether linkage is more stable than the structure of the polyester’s ester linkage. This means that the epoxy matrix will not be broken down by water as easily as the polyester matrix.  Epoxy has a service history proving its excellent resistance to blistering and other moisture related problems. For the custom boat builder, high-end production facility or marine repair yard, the high mechanical and chemical stability of epoxy, coupled with its excellent moisture resistance, make it an excellent choice to battle gelcoat blistering. 

Secondary Bonding 

There is one other compelling reason to use an epoxy resin rather than polyester resin. To effectively repair interlaminate failure, and to repair laminate surfaces damaged due to gelcoat blistering, the repair material must be a superior structural adhesive, capable of bonding to both polyester resin and the glass fiber. 

Unsaturated polyester resins perform fairly well during the construction of a structure when all of the layers of resin are applied and allowed to cure together. This type of bond is considered a primary bond. Problems can occur, however, when you try to bond polyester resin to a previously cured laminate as is necessary in blister repair applications. This type of bond is secondary or post-bonding. Epoxy, however, forms a superior bond with cured polyester in secondary bonding. Since the epoxy is stronger and shrinks less than polyester, the epoxy repair may actually be more durable than the original structure. 

Moisture exclusion ability, hydrolysis resistance, as well as secondary bonding capability are major considerations in choosing a barrier coating. If you also consider cost, ease and practicality of application, safety and access to technical assistance, Epoxy is an excellent choice for gelcoat blister repair. 

Recommendations for the Repair and Prevention of Gelcoat Blisters 

Inspect beyond the obvious 

If you plan to barrier coat your hull after repairing blisters, we feel it is important to inspect the hull beyond the obvious blisters. Before beginning repairs, we recommend grinding through the gelcoat in several small areas. These profile inspection points, 4" to 6" in diameter, can provide valuable information on the condition of the hull laminate. If the laminate shows signs of hydrolysis, consider removing all of the gelcoat and damaged laminate. It is pointless to barrier coat over a hull that has begun to deteriorate. Just as with skin cancer, the more serious problem may lie below the surface. 

Thoroughly Dry the Hull Laminate 

Open up all blister cavities and excavate damaged material. Insure that the laminate, throughout the hull structure, is as dry as possible. Active drying methods, such as heating and tenting, may be used to accelerate the process. 

Repair blisters and delamination with Epoxy products 

After sealing with a coat of unthickened epoxy, fill the cavities with an Epoxy thickened with either Microlight or Low-Density filler. In areas with extensive blister damage, rebuild the laminate with multiple layers of glass fabric bonded with epoxy. 

Apply barrier coat of Epoxy with a Barrier Coat Additive 

Thoroughly clean all underwater surfaces and then abrade by sanding, waterblasting or sandblasting. With the hull properly prepared, apply a minimum of 20 mils (0.020") of Epoxy to the surface. This can usually be applied in 5–6 coats. Up to 10 ten coats will provide added protection. Apply the first coat without any additives. The remaining coats should contain Barrier Coat Additive, which improves the epoxy’s moisture permeation resistance as well as its resistance to scratches and scuffs. 

Provide ventilation to all parts of the hull—keep the bilge as dry as possible 

Good ventilation is a key to the longevity of your boat. Water vapor can penetrate hull laminate faster than water in liquid form. One of the best recipes for creating a high-temperature humidity chamber is to leave your poorly ventilated, tightly sealed boat in the hot sun for weeks on end. Deck temperatures can exceed 130°F (55°C), pushing cabin temperatures toward 100°F (38°C). Such a rain-forest environment provides the necessary elements for gelcoat blistering since moisture can and will pass through either side of the hull laminate. In tropical climates, where heat and humidity are an extreme problem, you may want to consider having a dehumidifier aboard. Bilge water is also an obvious source of moisture, so it is important to keep the bilge as dry as possible. We strongly recommend active ventilation in bilge areas with powered vents, especially on boats that have previously blistered. 

Maintain the barrier coat’s integrity 

Excessive sanding during haul outs, groundings, scrapes and scratches will all undermine the moisture resistance ability of your epoxy barrier coat. Keep a high-quality bottom paint on the hull. Repair scratches, dings or abrasion damage as soon as possible, recoating the repaired area with epoxy to replace the removed barrier coat. 

After several haul outs, your barrier coat may have been reduced from repeated sanding. Consider removing the bottom paint and reapplying two or three coats of epoxy. Do not let blisters go unchecked. As soon as possible, repair the blisters and coat with epoxy to prevent further degradation. Monitor the hull’s moisture periodically. Early detection of moisture absorption can save you considerable expense and frustration in the long run. 

Repairing skin delamination 

Skin delamination is often first noticed when you step on a flexible or spongy area on an otherwise firm deck. Most delamination is a result of moisture damage to the skin/core bond and usually involves balsa core or plywood cored panels. Moisture entering cracks or nearby loose hardware can migrate much more easily through these cores than a foam core. Also, balsa and plywood cores are much less expensive and more widely used than foam cores in production boats. Often the core material will be wet or even saturated, but it may still be serviceable if dried thoroughly. However, if a wooden core remains wet long enough, it will begin to deteriorate and will need replacement. 

The core material separating the two skins of a cored panel reduces the tensile and compressive loads on the skins and allows a structural panel to withstand greater bending loads without a proportional increase in weight. To do this, the core material must remain bonded to both skins and be able to resist compression loads applied by the skins when the panel bends. Although the skin itself may not be damaged, it may be necessary to cut or remove a portion of the skin for access to the core.  

Types of core related damage 

Core related damage can vary from a small skin delamination with little or no actual damage to either the core material or skins, to moisture related core deterioration, to collision damage that can leave a hole through both the core and the inner and outer skins. The repair procedures in this section begin with the least damage, easiest to repair: 

1. Repairing skin delamination. Often the core is wet, but still firm and usable. A delaminated area may be several square inches or several square feet. 

2. Replacing damaged cores. The skin may be intact, but moisture penetration over time may have caused the balsa core to deteriorate. An impact may puncture the outer (or inner) skin and core without affecting the other skin. Even a minor puncture can allow moisture to migrate under the skin and affect strength of the core over a large area. 

3. Repairing transom delamination. The plywood core may delaminate or rot as a result of moisture penetration through a crack or hole in the transom skin. 

4. Repairing holed panels. An impact or modification can require rebuilding of the entire panel structure. Impact damage can extend to both skins and core or, one skin with major core damage. 

The work required to repair each type of damage varies with the size of the damaged area. Often, the full extent of damage cannot be determined until you have removed a portion of the outer skin. After a thorough inspection and assessment of the damage, follow the procedure that is most appropriate to your situation and keep in mind that the objective is to restore the structural properties of the panel by rebuilding the load carrying ability of the core and the skins to the original or greater strength. 

Delamination can also occur in isolated pockets as a result of inadequate core bonding during manufacture. In some cases, the core may remain dry and undamaged, and simply need re-bonding. 

Assessing delamination damage 

The first step in the repair is to determine the extent of delamination and the condition of the core. Then follow the repair procedure most appropriate to your situation. 

Locate and mark the extent of the damage by exerting pressure on the panel, checking for a soft feel and/or skin movement. Tap around the suspected area lightly with a small, hard object to help reveal the area of delamination. A void under the skin will sound flat or dull, compared to a more resonant sound of a solid laminate. 

When you push against the surface, the delaminated skin will give easily until it hits the core. If the core is solid, the skin will appear fair when it’s pressed tight against the core. If the core is damaged or deteriorated, you will be able to push the skin below the fair surface of the deck or hull. Water or air may squeeze from a nearby crack or hardware fastener. 

Determine the condition of the core material by drilling 3/16"–7/32" (5 mm)-diameter inspection holes through the skin several inches apart over the delamination. Push the skin tight against the core and drill through the core without drilling into the opposite skin. Collect the core material removed by the drill. Squeeze the core material tightly between your thumb and finger to determine whether the material is wet or dry and examine it for signs of decay. You may also insert a wire or nail through the hole to probe the core. If you hit voids or the core feels soft or punky, the core should be replaced. 

Re-bonding delaminated skin when the core is dry 

If the core material is firm and dry, re-bond the skin by injecting epoxy between the skin and core as follows: 

1. Cut 11/8"  from the tip of a syringe. Cut the tip at an angle. Fill the syringe with an epoxy filler mixture thickened to the consistency of ketchup.

2. Inject the epoxy mixture under the skin through each of the inspection holes. The shortened tapered syringe tip will fit tightly in the 3/16"–7/32"  inspection holes.

2. Inject the epoxy mixture under the skin through each of the inspection holes. The shortened tapered syringe tip will fit tightly in the 3/16"–7/32" inspection holes.

3. Clamp the skin to the core when you are sure you have injected enough epoxy to bridge any gaps between the skin and core. Use weights, braces or sheet metal screws through the inspection holes to hold the skin tight and fair against the core until the epoxy cures. Clean up excess epoxy before it begins to gel. Allow the epoxy to cure thoroughly before removing clamps. 

4. Fill any voids in the inspection holes with a thick epoxy filler mixture, or Epoxy Adhesive, after removing clamps. When the epoxy has cured thoroughly, fair and finish the surface.

Re-bonding delaminated skin when the core is wet 

If the core material is wet but still solid, re-bond the skin after the core has been thoroughly dried. One of two methods may be used to expose the core for drying. 

Pattern hole drying method 

This method involves drilling a pattern of holes through the skin to expose the core to air and heat and allow moisture to escape. When the core is dry, epoxy is injected under the skin and the skin and core are clamped together until the epoxy cures. This method is useful if the delamination is small, not under an area of non-skid deck or not in highly loaded or critical laminates such as hull bottoms. 

1. Drill 3/16"–7/32" diameter holes at 1"  intervals, creating a pegboard-like pattern that extends several inches beyond the delaminated area. The holes should penetrate the fiberglass skin and the core without drilling into the opposite skin. Use a drill depth control device to prevent drilling entirely through the panel.

2. Dry the core thoroughly. If the core is extremely wet, start by using a high-powered shop vacuum cleaner or vacuum bagging to draw water out of the laminate. A heat lamp or radiant heater with some air movement over the area will speed the drying. 1"  apart over the area of delamination, to allow the core material to dry out. 

3. Cut 11/8" from the tip of a Syringe. Cut the tip at an angle. Fill the syringe with a Resin based epoxy mixture thickened with a filler to the consistency of ketchup. 

4. Inject the epoxy mixture under the skin through each of the holes starting in the center of the delaminated area. The shortened tapered syringe tip will fit tightly in the 3/16"–7/32" (5 mm) holes. You should be able to develop enough pressure to force the epoxy to the surrounding holes.

5. Clamp the skin to the core when you are sure you have injected enough epoxy to bridge any gaps between the skin and core. Use evenly placed weights or braces covered with plastic to hold the skin tight and fair against the core. Sheet metal or drywall screws will also work. Whichever clamping method you use, don’t distort the panel by applying too much pressure. You only need to keep the skin fair and keep the skin and core in contact while the epoxy cures. Remove the excess epoxy before it begins to gel. Allow the epoxy to cure thoroughly before removing clamping. 

6. Sand the surface and fill any remaining holes with an epoxy low-density filler mixture, thickened to the consistency of peanut butter. After the epoxy has cured thoroughly, sand the surface fair.

For thin skins, this procedure may result in a weakened structure, making it necessary to bond several layers of 6 oz. fiberglass fabric over the repair area.

Skin removal method 

This method involves removal of a section of skin to expose the core for drying. Because of the difficulty (or impossibility) of fairing and finishing a non-skid surface, it’s often easier to cut and remove an entire non-skid area. After the core is dried, the skin is re-bonded and then patched and refinished at the smooth areas outside of the nonskid.  

Replacing damaged cores 

This method is recommended when the core is damaged and must be replaced to restore the original strength and stiffness of the laminate and when a skin delamination is in a non-skid area of a deck. After the core is replaced or dried, the skin is re-bonded to the core and a repair patch is laminated over the joint to restore skin continuity. 

Remove the skin and replace the damaged core as follows: 

1. Cut through the skin around the area of delamination with a panel or circular saw with a carbide tipped plywood blade or a router with a small diameter straight-fluted bit. Set the blade or bit to the depth of the skin only. On smooth surfaces cut several inches outside the area of delamination. If the delamination is in an area of non-skid, cut in the smooth area several inches outside of the non-skid area or midway between non-skid areas. 

2. Remove the skin. The skin should separate easily in areas where the core is damaged or wet. In areas where the skin is well bonded to the core, use a chisel or thin blade between the skin and core to pry the skin away from the core material. Be careful not to bend the skin too much or gouge the core. Applying heat to the joint with a heat gun will also help to soften the skin core bond. Be careful not to overheat the skin. 

3. Dry the core thoroughly. If the core is extremely wet, start by using a high-powered shop vacuum cleaner or vacuum bagging to draw water out of the laminate. A heat lamp or heat gun will speed the drying. If the core is undamaged, skip the core replacement and re-bond the skin.

4. Remove damaged core material. Cut around the area of damage with a utility knife. Use a chisel to remove the damaged core and shave all traces of core from the opposite skin. 

5. Prepare a new piece of core material to match the shape, thickness and density of the core that was removed. Dry fit the piece to be sure the new piece is no higher than the surrounding core. When replacing damaged core material, try to purchase the same material used by the builder. If that is impossible, locate a material as close as possible to the core’s original thickness and density. It is better to have a slightly thinner core material than a thicker one. 

If the damaged area is smaller than about 2"×2", the area may be filled with a thick epoxy filler mixture. If the core is over 1" thick, fill the void in multiple layers, allowing each previous layer to gel, to avoid exotherm. 

If the damaged area is smaller than about 12"×12" and the original core material is not available, you may substitute core material cut from soft woods like pine, fir or cedar. Cut short blocks to the length of the appropriate core thickness. For example, standard fir 2×4’s, cut to 1⁄2"  lengths, will yield 11⁄2"×31⁄2" end-grained blocks that may be trimmed to fit like tiles in place of damaged 1⁄2" core. 

6. Wet out the contact surfaces with a Resin/hardener mixture. Then apply a liberal coat of Resin/hardener mixture, thickened to the consistency of mayonnaise with filler, to one contact surface. Apply enough thickened epoxy in an even layer to bridge all gaps between the two surfaces. This alternative is recommended for larger repairs. Or, apply a liberal amount of Epoxy Adhesive, dispensed through the static mixer, to the core contact area. Spread the adhesive to a layer thick enough to bridge all gaps between the two surfaces. 

7. Press the core material firmly in position. A small amount of thickened epoxy should squeeze from the joint around the piece. Clamp the piece (or pieces) with plastic covered weights or braces, if necessary, to hold it in place. Smooth the epoxy at the joint and remove excess epoxy before it begins to gel. Allow the epoxy to cure thoroughly before removing clamps. 

Re-bonding the skin 

If the skin was damaged from an impact or abrasion or damaged during removal, laminate a new skin in place against the new core. If the skin is reusable, re-bond the skin as follows: 

1. Sand the surface of the core and the inner surface of the skin that was removed. Dry fit the skin for fit and to be sure that it lays flat and fair with the adjoining skin. 

2. Wet out the contact surfaces with a Resin/hardener mixture. Then apply a liberal coat of Resin/hardener mixture, thickened to the consistency of mayonnaise with filler, to one contact surface. Apply enough thickened epoxy in an even layer to bridge all gaps between the two surfaces. This alternative is recommended for larger repairs. Or, apply a liberal amount of Epoxy Adhesive, dispensed through the static mixer, to the core contact area. Spread the adhesive to a layer thick enough to bridge all gaps between the two surfaces. 

3. Push the skin to the core in its original position with an equal gap (the saw cut)around all sides.  A small amount of epoxy should squeeze from the joint. Clamp the skin in position with vacuum bagging, weights, braces or sheet metal screws. Remove excess epoxy before it begins to gel. Allow the epoxy to cure thoroughly before removing clamps. Vacuum bagging is an alternate clamping method that provides equal clamping pressure over large areas. 

4. Grind a 12-to-1 bevel on both edges of the joint and laminate a repair patch over the joint to restore skin continuity. The bevel will provide a recessed bonding area for the application of fiberglass fabric and allow the patch to be faired flush with the surface. The objective and procedure for patching the joint are the same as repairing damaged skins. 

Replace damaged core from below 

A variation of this technique is to replace damaged core by removing the lower or inside skin and leaving the outer skin intact. Although working overhead makes this variation more difficult, the difficulty is offset by the elimination the cosmetic work that would be required on the outside of the deck. This variation is especially useful when the repair is in a non-skid area of the deck. 

The underside of a deck is often covered by a fabric liner, which can be peeled back to expose the repair area and then put back in place, hiding the repair.  After removing the skin and the damaged core, replace the core as described above.  Except, cut the core into convenient sizes for working overhead and use epoxy thickened enough to hold the pieces in place overhead. Immediately bond the inner skin to the core and, as necessary, use sticks to prop the entire repair in place, flush with the surrounding skin, until cured.  When the repair has cured, it is not necessary to grind a bevel to make the repair flush with the surface.  With the proper fabric overlap on each side of the joint (12 times the thickness of the laminate), a structural repair can be achieved without grinding a bevel.  Prepare both sides of the joint to achieve a good bond and apply the layers of fabric over the joint as you would if you were applying to the bevel.  When the epoxy has cured, put the liner back in place and the repair is complete. 

Repairing transom delamination 

Removing a section of skin to expose and replace the core is a method often used to
repair delaminated powerboat transoms. Transoms are major structural parts of fiberglass powerboats, especially outboards. They not only support the weight of the motor, they maintain the shape of the boat and are a mounting point for hold-downs, towing eyes and other accessories. 

Outboard motors apply a considerable load to the transom. The effects of the motor’s weight are concentrated on small areas of the skin and core when the boat is accelerated under normal operating conditions, and when the boat is bouncing along on a trailer.

Over time the core is crushed and cracks develop in the skin. Moisture penetrates the plywood core, leading to delamination and eventual rot. Moisture can also penetrate the transom skin at hardware fasteners and around drain holes and I/O cutouts. All of these factors have a cumulative effect on structural failure. 

Excessive motor movement may be your first sign of trouble. Tap around the suspected area lightly with a small, hard object to help reveal any areas of delamination. A void under the skin will sound flat or dull, compared to a more resonant sound of a solid laminate. Damage can be confirmed by drilling 3/16" –7/32" diameter inspection holes into the core at the suspected delamination. Examine the core material removed by the drill for signs of water or decay. 

Planning the repair 

The objective is to remove and replace the damaged core. Access to the damaged core is gained by removing the fiberglass skin from either the outside or, if possible, the inside of the transom. The boat’s design determines which method is more practical. Interior access requires much less cosmetic finishing, but stringers, soles or decks often make interior skin removal impossible. The following method describes accessing and removing the core from the outside. If the removed fiberglass skin is in good condition, it is usually glued back in place over the new core, then structurally and cosmetically blended into the surrounding skin. Support the hull to prevent sagging or distortion before removing the skin and core. 

This repair is much easier if the total area affected can be confined to the transom and not extend around the corners. It is much easier to end a paint job at the corner of an object, where there is a visual break, than it is to match color and texture in the middle of an area. The following transom repair method leaves enough fiberglass around the perimeter of the transom for a proper bevel and repair patch, yet allows enough access for the damaged core to be removed and replaced. Before making a substantial cut through a structural fiberglass skin, support the hull with blocking to maintain the hull’s shape. 

Determine the location of the cut 

After removing the motor, hardware and trim, measure the fiberglass skin thickness at one of the holes in the transom. The fiberglass thickness determines the bevel length and the distance of the cut line in from the corners. The bevel length is at least 12 times the fiberglass thickness. 

A 12-to-1 bevel allows room for multiple layers of fiberglass fabric and epoxy across the cut line to restore strength to the fiberglass skin. If the fiberglass skin is 1/8"  thick, the width of the bevel will be at least 11⁄2". If the fiberglass is 1⁄4" thick, the cut line will need to be at least 3"  in from the corners to allow for a 3" bevel. If the original fiberglass skin is reused, the same bevel is required on each side of the cut after the skin is re-bonded to the new core. 

Layout the cut line on the transom the required distance from the edges. Measure from the end of the rounded corners where they blend into the flat transom surface. 

Removing the damaged skin and core 

The skin removal method as described above for deck or hull panel delamination is essentially the same for transom delamination. Often, if the core has been wet for a period of time, the plywood veneers will begin to delaminate and much of the veneer may have rotted away. Remove the outer (or inner) transom skin and plywood core as follows: 

1. Cut through the skin at the cut line established by the bevel length. Use a panel or circular saw with a carbide tipped plywood blade or a router with a small diameter straight-fluted bit. Set the blade or bit to the depth of the skin only. 

2. Remove the skin. The skin should separate easily in areas where the core is damaged or wet. In areas where the skin is well bonded to the core, use a chisel or thin blade between the skin and core to pry the skin away from the core material. Applying heat to the joint with a heat gun can help to soften the skin core bond. Be careful not to overheat the skin. Avoid damaging the skin by over bending or using too much force when prying the skin from the core. It is worth the effort of getting it off in one piece. 

3. Inspect the condition of the plywood core material. If the plywood is sound but wet, you may be able to dry it and re-bond the skin. Drying a saturated transom core thoroughly may take weeks or months. Unless you have plenty of time to allow for drying, over a winter season for example, you may consider replacing all the core. If you are able to dry a sound core, fill any minor voids or delaminations or end grain in the dried core while it is uncovered. However, the plywood has deteriorated or dry rot has set in, the plywood should be replaced. Even if the core damage is isolated to some portion of the transom, you may want to consider replacing all of the plywood core rather than repairing it. 

4. Remove the damaged plywood core. Use a chisel or whatever combination of tools you need to remove all damaged material. The plywood core in the perimeter and corners of the transom will be the biggest challenge. If necessary, use large drill bits to carefully weaken and re-move stubborn areas. Rotary rasps can also be effective. Home made tools similar to a grub how can be used to remove the stubborn perimeter areas. Shave all traces of the core material from the inner skin, being careful not to damage the skin. Make any necessary repairs to the inner skin. Sand the skin to prepare for bonding. 

Preparing a new transom core 

Prepare a new plywood core to match the shape and thickness of the core that was removed. Try to use the same grade plywood (or better) that was used in the original core. Marine grade plywood is ideal for this repair. If marine grade plywood is not available, use more layers of thinner AB grade exterior plywood. Be sure to fill any voids in the plywood edges with thickened epoxy after the panels are cut into shape. 

Because the opening is smaller than the core we are replacing, the plywood will have to be installed in multiple layers made up of smaller pieces of the same type of plywood, laminated to equal the thickness as the original core. For example, if a 11⁄2"thick core consisted of two sheets of 3⁄4"  plywood, it would be better to laminate three sheets of 1⁄2"  or four sheets of 3/8"  to equal the original 11⁄2" core thickness. Trim and dry fit the new pieces of core to fit the void left by the old core. 

Make a template of the transom core and use it to layout the plywood layers. Because the opening in the transom skin is smaller than the full sized plywood core, you will need.to replace each layer of the core in pieces. Cut each layer into pieces small enough to fit through the opening in the transom skin. Stagger the joints in each layer, by at least eight times the plywood thickness, from joints in the adjoining layers. For example, a 3" stagger will be required between joints when using 3/8"  plywood. Ideally these joints are staggered widely. Joints can also be run at different angles to stagger them even more. 

Joints near the sides of the transom will affect strength less than joints near the middle. Remember, the cantilevered load of an outboard motor puts significant loads on the middle of the transom. If joints in the layers are scarfed with an 8:1 bevel rather than butted, joint location is not an issue. 

Before mixing epoxy: 

Plan all of the installation steps. The layers can be glued in place in one continuous operation or over several sessions.
 

Label the pieces and dry fit them in the transom to eliminate potential problems during actual assembly. The fit need not be perfect—thickened epoxy will bridge gaps.
 

Use Slow Hardener for extra working time. Use Extra Slow Hardener if you will be working in warm temperatures.
 

Be sure you and any parts of the boat you do not want to get epoxy on are protected.
 

Be sure all parts, tools and clamps are within easy reach. Drywall screws are a practical clamping method for a plywood transom lay-up. Clamps, wedges or prop sticks can also be used. Bolts with nuts and oversized washers can be used in places where holes will eventually be required—motor mount holes and drain holes.  

Installing the new core
If you will be working alone and wish to accomplish the repair in manageable steps, laminate the new pieces of plywood core in place as follows:
 

• 1. Prepare all bonding surfaces. Check the condition of the inner skin and make repairs as necessary. Sand rough surfaces. Remove all loose material and dust.
   
• 2. Wet out the bonding surfaces of the inner fiberglass skin and first piece of core with epoxy. Apply extra epoxy to the plywood end grain.
   
• 3. Coat the bonding surfaces of the inner skin and hull edges with an epoxy mixture thickened with a filler to the consistency of mayonnaise. Use a notched spreader to apply enough thickened epoxy to bridge all gaps between the core piece and the skin and the edge of the core and the hull. Avoid over-thickening. The epoxy needs to move under minimum pressure. Adjust viscosity by adding more or less filler to achieve consistency between that of catsup and mayonnaise.
   
• 4. Place the first piece of plywood in position against the coated skin. Clamp the piece in place with drywall screws (and oversized washers) through the inner skin to draw the pieces of core tight to the inner skin. Coat the screws with a mold release for easy removal. A small amount of epoxy should squeeze from the joint around the core. (Fill the screw holes with epoxy after the lay-up has cured and the screws removed.)
   
• 5. Repeat the process for the remaining pieces of the first layer. Fill any gaps and smooth the epoxy at the joints. Remove excess epoxy before it begins to gel. Allow the epoxy to cure thoroughly before removing screws, clamps or vacuum pressure. Once cured, you have a rigid base against which you can glue or laminate the remaining plywood one layer at a time.
   
• 6. Recheck the fit of the final pieces and be sure to sand cured epoxy coated surfaces prior to laminating additional plywood layers.
   
• 7. Repeat the process for each layer. Use drywall screws to draw the new layer down to the first layer. Remove the screws and fill the holes with epoxy after the epoxy cures. You may leave the fasteners in place only if they are stainless, galvanized or bronze. 

An alternative method is to install and clamp all the layers in place at the same time if
a very slow curing hardener is used. Dry fit and label all of the parts. Apply unthickened epoxy to all of the plywood segments, especially the end grain around the perimeter of each piece, before applying thickened epoxy. Pieces can be temporarily clamped with drywall screws. Remove and replace the screws as each layer is installed. You may also use C-clamps, prop-sticks braced against a wall or bolts and oversized washers in holes that will be used for through-hull fittings. 

Replace the fiberglass skin 

After the core replacement is complete, the original fiberglass skin can be laminated over the new core or if necessary a new skin can be laminated over the core. A transom skin is often reusable except for a relatively small damaged area around the motor mount. If the damage is limited, it may be easier to repair the damage in the center of the skin, after re-bonding the skin, rather than laminating a new skin over the entire transom. Re-bond the original skin as follows: 

1. Sand the bonding surfaces of the core and the skin. If the skin was repaired, be sure the back side of the repair is sanded flush. Dry fit the skin to be sure it lays flat and fair with the adjoining skin. 

2. Wet out the bonding surfaces of the core and skin with epoxy. Bond the skin to the core in its original position using the same laminating techniques used to bond the plywood core in place. Alternatively, you may use Epoxy Adhesive to bond the skin in place. 

3. Clamp the skin with drywall screws driven through plywood blocks or oversized washers. Large blocks or washers spread the holding power of the screws over a larger area and prevent dimples in the fiberglass skin that would require filling and fairing later. Use pieces of plastic sheet or plastic packaging tape under the washers or blocks to prevent bonding to the skin. Allow the epoxy to cure. Remove the screws, bevel the edges of the holes and fill them with epoxy. 

4. Grind a minimum 12-to-1 bevel on both sides of the joint. The outside edge of the bevel should be short of the corner. Laminate a repair patch over the joint to restore skin continuity. The bevel will provide a recessed bonding area for the application of fiberglass fabric and allow the patch to be faired flush with the surface. The procedure for patching the joint are the same as repairing damaged skins.

Laminating a new transom skin 

If your fiberglass skin is unusable, plan to laminate a new fiberglass skin over the core with multiple layers of fiberglass and epoxy. Apply enough layers to equal the same thickness as the original skin. Layers can be applied immediately or while the previous layer is still tacky. If the epoxy is allowed to cure beyond being tacky, allow it to cure overnight, then wash the surface with water and sand the surface to prepare it for more layers. Laminate the new skin so it extends to the edge of the 12-to-1 bevel that was machined earlier on the outer edges of the transom. Install the biggest patch first with each layer being progressively smaller to fill the 12-to-1 bevel. For additional information on laminating a large new fiberglass skin.

1. Grind a 12-to-1 bevel around the remaining edge of the transom. The outside edge of the bevel should be short of the corner.

2. Cut the appropriate number of fabric pieces to the size and shape of the transom. Cut the first piece 1⁄4" from the outer edges of the bevel. Cut each of the remaining pieces smaller on each side than the piece below it. The final piece should be the same size as the inner edge of the bevel. The edges of the middle pieces should be evenly spaced between the edges of the first and last pieces. The spacing depends on the number of pieces (equaling the laminate thickness) and the length of the bevel. 

3. Wet out the transom core and bevel with epoxy. Using a plastic spreader, coat the transom core with a thin even layer of an epoxy mixture thickened with filler to a mayonnaise consistency. 

4. Wet out the cloth with epoxy on a work surface covered with plastic sheeting. Apply enough epoxy to saturate the cloth using a thin foam roller or by pouring epoxy onto the fabric and spreading with a plastic spreader. 

5. Place the fabric on the transom, centering it within the edges of the bevel. Smooth the fabric into the thickened epoxy layer with a plastic spreader. Use the spreader to remove trapped air and excess epoxy and to smooth the fabric against the surface. 

6. Wet out and apply each of the remaining fabric layers with epoxy, ending with the smallest piece. Apply each layer before the previous layer becomes tack free. Smooth each layer with a plastic spreader to remove wrinkles, trapped air and excess epoxy. 

7. Apply several coats of epoxy over the fabric to fill the weave of the fabric when the epoxy has reached a gel stage. Use the thin foam roller to apply each coat after the previous coat reaches its gel stage and before it becomes completely tack free (to avoid sanding between coats). Allow the final coat to cure thoroughly. 

8. Wash and sand the final coat after it has cured thoroughly. Sand and fair in the edges of the fabric to blend with the hull surface. Any flaws or unevenness can be faired with a thick mixture of epoxy or Microlight. Seal filled and sanded surfaces with epoxy and wet sand to prepare the surface for paint. 

Be sure to coat and seal the end grain of all holes drilled through the transom with multiple coats of epoxy. This is not a waste of time. If the holes are not properly sealed, plan on replacing the core again in the future. If all sources of water are eliminated by sealing the wood in epoxy, the repair should be better than new and last indefinitely. Remember to coat screw holes with epoxy just prior to installing screws for motor mounts and transom hardware. Apply mold release (or vegetable oil cooking spray) to the fasteners prior to gluing them in place if you plan to remove them at some point in the future. 

Replacing the core and both skins 

If you can’t separate the skins from the core or if the skins and core are damaged beyond repair, cut the entire transom section out, and bond in a new core as follows: 

1. Cut away the transom by sawing through the skins and core at the perimeter of the transom. Prepare the bonding area by sanding away any rough or uneven edges around the hole perimeter. Be sure the hull is supported to prevent distortion. 

2. Prepare the transom core replacement. Bond the plywood pieces together on a flat surface or a surface matching the curve of the transom. Trim the perimeter of the new core, checking the piece for fit in the exact position of the old core. Be sure the hull is set up square and true before bonding the new transom in place. 

3. Bond the new core in position. Wet out the bonding surfaces of the new plywood core and the hull with epoxy. Use particular care to thoroughly wet out the end grain of the plywood. Coat the bonding surfaces of the core and hull with an epoxy mixture thickened with filler to the consistency of mayonnaise. Use enough of the mixture to ensure no voids remain between the two surfaces when the core is positioned. 

4. Clamp or brace the core in position until the epoxy cures. Use only enough clamping pressure to restrict movement and squeeze out some of the mixture. Remove the excess thickened epoxy or use it to fill any voids in the joint before it begins to gel. Align the core in the exact position of the old core and allow the epoxy to cure thoroughly before removing clamps. 

5. Re-bond reusable skins.

Repairing holed panels 

Minor impacts or abrasions can include dropping a cooler or sharp tool on the deck, rubbing against a dock or the weight of an outboard motor mount against a transom. This type of damage often results in a hole or crack through one skin and possible skin delamination around the impact. If the damaged area has been submerged or left unprotected for a period of time, water penetration can lead to further delamination and eventually rot. Most minor impact damage can be repaired with procedures previously described. 

Major impact damage is often associated with collisions, groundings, natural disasters and occasionally traffic accidents. The amount of damage depends on the force of the impact and the shape of the object being hit. In this case, we are referring to structural damage through the core and both skins. Damage may mean a fracture that results in loss of skin continuity or a hole several feet across. There is also a strong possibility the impact that damaged the panel may have caused internal structural damage to bulkheads, frames, etc. Any such damage should be repaired prior to undertaking the repair of the cored panel. 

The objective in repairing holes through cored panels is to replace damaged core material and restore skin continuity to both skins. The sequence of skin and core replacement will vary depending on access to the back of the panel. 

Repairing holes through cored panels with back access 

After preparing the hole, replace the core material and laminate the inner and outer skin to the core as follows: 

1. Prepare the hole by cutting away ragged or damaged skin. Cut back to undamaged core and skin while maintaining a circular or oval hole shape. Grind a minimum 12-to-1 bevel on the edges of the inner and outer skins at the hole to provide adequate bonding surface when re-laminating the skin repair patches. For example, if the skin is 1⁄4" (6 mm)-thick, the bevel will extend 3" (76 mm) from the edge of the hole.

2. Prepare a temporary backing to support the core while bonding it in position and while laminating the new outer skin patch. Cut a piece of low-density insulating foam slightly larger than the hole opening. Bevel the edges of the foam so the face of the foam is flush with the inner skin/core bond line. Cover the foam with 4-6 mil plastic and brace it in position against the inner edge of the hole. If the foam is too stiff to conform to a curved panel shape when braced from the back, sand the foam to a shape that matches the panel curve.

3. Prepare a new piece of core material to match the shape, thickness and density of the core that was removed. Dry fit the core to match the shape and contour of the core that was removed. When replacing damaged core material, try to purchase the same material used by the builder. If this is impossible, locate a material that is as close as possible to the core’s original thickness and density. Cut the core into smaller pieces as necessary to conform to curved panels. 

4. Wet out the edges of the hole and edges of core material with epoxy. Apply a layer of epoxy mixture thickened with filler to the consistency of peanut butter to the inside edges of the hole and edges of core material. Alternatively, you may apply a layer of Epoxy Adhesive to edges of the hole and core or core pieces without wet out. 

5. Place the core material in position. If the epoxy mixture will not hold the core in place or if it will not conform to the panel contour, use braces to hold it in position. Smooth any excess epoxy at the joint and fill any voids in the joint before the epoxy begins to gel. Allow the epoxy to cure thoroughly. 

6. Laminate a new outer skin repair patch.

7. Laminate a new inner skin repair patch after removing the temporary backing support. 

Repairing Core Related Damage Repairing holes through cored panels without back access 

The difference between repairing holes with or without back access is in the sequence of steps. Without back access it’s necessary to laminate the inner skin first, replace the core, and then laminate the outer skin. Laminating the inner skin from the outside requires additional preparation, as follows: 

1. Prepare the hole by cutting away ragged or damaged skin. Cut back to undamaged core and skin, while maintaining a circular or oval hole shape. 

2. Cut back the outer skin and core several inches from the edge of the hole through the inner skin to provide an area wide enough to grind a 12-to-1 bevel around the edge of the inner skin. Use a router with a straight fluted bit set to the depth of the outer skin and core only, to avoid cutting the inner skin. Cut the core and outer skin back 12 times the thickness of the inner skin plus about 1" (25 mm) to leave enough room to grind the inner skin bevel. For example, if the inner skin is 1⁄4" (6 mm)-thick, the bevel will extend 3" from the edge of the hole. The core and outer skin should be cut back about 4" from the edge of the hole through the inner skin. 

3. Grind a minimum 12-to-1 bevel around the inner edges of both skins to provide adequate bonding surface when re-laminating the skin repair patches.

4. Bond a permanent backer to the back side of the inner skin.The backer should match the contour of the panel. 

5. Laminate a new inner skin repair patch.

6. Bond a new piece of core material in place against the new inner skin. 

7. Laminate a new outer skin repair patch. Finish the outer skin.

Hull Preparation 

This section covers the procedure for removing damaged gelcoat and laminate, and for abrading the surface to prepare a hull for drying, filling, fairing and final moisture barrier coating. 

The probability for the success of this repair, and the prevention of future blistering, depends on a variety of factors, many of which are beyond your control. These include quality control during the hull’s manufacture, the quality of raw materials used in construction, the age of the boat, and the climate it was exposed to. Because of factors such as these, it is impossible to absolutely rule out future blistering. 

Evaluating blister damage  

If at all possible, arrange to be present when the boat is first pulled from the water. Blisters tend to shrink quite rapidly once the boat is out of the water, and can actually disappear within hours, only to reappear when the boat returns to the water. After cleaning off marine growth and dirt, scuff the bottom with 80-grit sandpaper. Blisters will show up as light spots against the darker bottom paint. Damage may range from a few large isolated blisters to an entire hull peppered with thousands of blisters no bigger than a pencil eraser. Damage may also vary considerably from one area of the hull to another. Closely inspecting the hull as soon as it’s pulled will allow you to more accurately assess the nature and severity of the blistering and choose the best course of repair. 

A thorough inspection includes evaluating the laminate below the gelcoat. Grind 4"–6" diameter inspection points in several places on the hull. Use a disk sander to grind shallow concave areas through the gelcoat. Wet the areas with water or alcohol. When wet, a healthy laminate looks dark and translucent. If white fibers are evident, at or below the surface, it is an indication of a manufacturing defect or that resin around the fibers has degraded and hydrolysis has taken place. If there appears to be laminate damage, grind additional profile inspection points to confirm it and determine the extent of the damage and possibly how much laminate will have to be removed. 

Isolated minor blisters can be opened individually and the individual blister cavities filled and faired. This method has worked well, especially on older boats. Since older boats have survived for some time with only limited damage, it is often the case that little damage exists beyond the apparent blisters. 

Extensive damage on newer boats, however, may indicate a serious material or manufacturing defect. In these cases the gelcoat should be completely removed. This eliminates the obvious damage as well as any blistering in its early stages, and it allows you to inspect the laminate for defects or damage. The gelcoat will then be replaced by the epoxy barrier coat. This option is a good idea, if time and funds allow, even if the blistering is not yet serious. 

In either case, open the blisters and abrade the hull or remove the gelcoat as soon as possible. This will allow the cavities, the remaining gelcoat and the laminate to dry much more quickly. Removing the gelcoat will allow the laminate itself to dry out quicker since the moisture will not have to travel though the gelcoat. Thorough drying is an important and often rushed part of the repair. Opening blisters or removing the gelcoat quickly, with frequent washing during the drying process, will get the most out of a limited drying time. 

Be sure to wear protective clothing and eye protection when opening blisters. The acidic blister fluid is frequently under as much as 200 psi of pressure. When the blister is punctured, the fluid may squirt out with surprising force. 

Minor isolated blister damage

Keeping in mind that minor blistering may be a symptom of a larger problem, there are many situations where repairing isolated blisters makes sense. You may be able to repair isolated blisters and improve ventilation, and thus avoid the need for a full-blown barrier coat job. You may repair and monitor blisters for two or three years to determine the severity of your problems before decide on the most effective repair strategy. Or, you may simply want to get your boat into the water so you can go boating now. 

If your hull has a manageable number of blisters or blistering is limited to a small section of the hull, this technique allows you to repair blisters in a matter of hours prior to applying bottom pant. 

1. Mark the individual blisters or blister areas (by scuffing the bottom paint with 80-grit sandpaper) for future reference if you are not able to open the blisters right away, or: 

2. Immediately open the blister cavities. Use a small sanding disk (such as 3M’s RolocTM 2" diameter sanding disk) with 60-grit sandpaper, chucked into a variable-speed drill. Make sure that you have removed the entire blister, including the edges of the blister dome. 

Extensive blister damage 

If you’ve pulled the boat from the water and find yourself faced with hundreds or thousands of blisters, it may not be practical to open each blister individually. The job of filling and fairing all of the individual blister cavities also takes on monstrous proportions. Grinding, sandblasting or peeling are options that allow you to open all of the blisters and abrade the entire hull (or remove the gelcoat entirely) in one operation. Each method has advantages and disadvantages. 

Grinding 

Grinding or sanding is the most common method for opening blisters and abrading the gelcoat and is often the only option for boat owners who choose to repair their own blisters. However, grinding is not a pleasant job and, depending on the skill of the operator, may leave the hull uneven and in need of extensive fairing. The operation also creates a lot of dust and a potential health hazard. The equipment required is relatively inexpensive and widely available. It includes a 1000–2000 RPM air or electric polisher, with an 8" foam sanding pad. 

Grind the hull to remove bottom paint, open blisters and abrade the surface as follows: 

1. Prepare the work area to protect against dust hazards, especially when bottom paint is to be removed. Check local ordinances for restrictions and follow safe waste management practices. It is a good idea to remove all thru-hull fittings. 

2. Clean the hull of all marine growth and contaminants like grease or oil. 

3. Grind the hull beginning with a coarse grit (24–40) to strip the bottom paint and open blisters. Hold the grinder at a low angle (nearly flat against the hull) to avoid gouges. Remove enough of the gelcoat to expose all of the blister cavities or, if the blisters are shallow, continue removing gelcoat until the surface is flush with the bottom of the cavities. Leaving cavities in the surface will require more filling and fairing. In all cases, keep grinding until a solid, undamaged surface is exposed. 

4. Grind the surface again with a finer grit (50–80) to remove the coarser grit scratches and fair the surface. If sanded fair enough, little filling and fairing will be required. For this operation, an air file or double action (DA) sander may provide more control for fairing than the disc sander. You may chose to sand through all of the gelcoat to the first layer of laminate. 

5. Inspect the hull for any further damage after sanding. Sound the hull to detect any interlaminate voids. Repair any voids that are found before beginning the filling and fairing operation. 

Sandblasting 

Sandblasting (or water-blasting) involves much more expensive equipment which may
be rented. Professional sandblasting services are available in many areas. Sandblasting will leave a generally fair but pitted surface. This method is also a potential health problem because of the airborne dust generated. Because sharp sand is blasted into the surface under air pressure, bottom paint should be removed before sandblasting. Failing to do so may result in small particles of paint or contaminates being imbedded in the laminate, a condition which may create bonding problems later on. 

When using this method, be very careful not to sandblast too deeply. Do not drive gelcoat coat particles into the softer, underlying laminate nor remove excessive amounts of the laminate. Consider sandblasting 80 or 90% of the gelcoat away, and then finishing the process by sanding. After the gelcoat is removed, inspect the laminate for hydrolysis, delamination or other damage. 

Gelcoat peeling 

Gelcoat peelers are designed around an electric or hydraulic powered cutting head that shaves the gelcoat down to the appropriate depth in one pass. Peeling leaves a relatively smooth surface, which reveals flaws within the laminate better, and requires much less fairing than a ground or sandblasted surface. Peelers also allow for better waste collection than grinding or sandblasting, minimizing environmental hazards. Gelcoat peelers are relatively new technology and the service, where available, may be the most expensive of the options. However, if you are hiring the labor to sand your hull, peeling the gelcoat can be cost effective, especially if you consider the additional time and expense of fairing the sanded hull. After the gelcoat is removed, inspect the laminate for hydrolysis, delamination or other damage. Wash and sand the surface thoroughly before barrier coating. 

Sounding the hull 

You may find it a worthwhile investment of time to “sound” the entire hull. Wet or delaminated areas will sound dull or flat when rapped with a small mallet. Dry, solid laminate will have a sharp sound. By tapping the hull in a regular pattern every 3" (7.5 cm), you should be able to isolate problem areas. Blistering or delamination voids within the laminate may affect the structural integrity of the hull and should be repaired. 

Exposing and removing interlaminate damage 

After grinding or peeling away the gelcoat, wet the surface of the exposed laminate with water will help you to see flaws below the surface. Voids appear as lighter areas within the darker solid laminate. Voids may be the result of hydrolysis or a manufacturing flaw, which may show up in a pattern that reflects the pattern of resin application. Sounding will confirm the presence and extent of the voids. The deeper or more widespread the delamination, the more serious the structural problem. If sounding or visual inspection reveals voids below the outer layer of laminate, open the voids by drilling or grinding to allow the laminate behind the voids to dry out thoroughly before repairs are be made. 

Small voids 

If the area of a void is limited to a few inches, you can repair it without removing the outer layers of laminate. Drill a pattern of 3/16" diameter holes over the area of the void. Drill through the outer laminate without drilling past the void into the laminate below. The holes will allow you to evaluate the size of the void, the soundness of the laminate below the void and help the void to dry out. 

Dry the laminate

Large voids 

If larger areas of delamination are confined to the first layers of laminate or to limited areas of the hull, you can restore the structure by removing the damaged material and bonding in new layers of fiberglass cloth with epoxy. 

1. Mark the location and area of all of the voids with a felt marker. 

2. Grind out all of the damage, exposing solid undamaged laminate. 

3. Bevel the edge of the repair area to a minimum 12-to-1 angle to provide a greater bonding area and reduce stress concentrations

. 

After drying thoroughly, layers of fiberglass cloth must be bonded (laminated) over the repair area to restore the laminate to its original thickness and strength.

Evidence of hydrolysis within the laminate may appear as “fiber whiting.” In normal or healthy laminate the fabric fibers below the surface are translucent and unnoticeable. If the resin around the fibers has hydrolyzed, the fibers appear white and are noticeable through the laminate after wetting the surface with water. 

1. Inspect the hull to determine the extent of the damage after the gelcoat is removed. 

2. Grind or peel to remove all of the hydrolyzed material in the outer layer of laminate. This may include isolated areas or the entire area of the hull. 

3. Inspect the layer below the area(s) that were removed and (if necessary) remove all of the hydrolyzed material. Repeat the procedure one layer at a time until all hydrolyzed layers are removed. 

Dry the hull thoroughly and bond on (laminate) new layers of fiberglass cloth to replace any roving or other structural layers that were removed. The outer layer of chopped strand mat is not usually considered a structural layer. 

Note! If you are unsure of the extent of damage or question the soundness of the hull, it’s a good idea to get professional advice before attempting repairs. In severe cases, laminate analysis by a composites lab is advisable. Labs can analyze the laminate layer by layer for resin content, evidence of hydrolysis and moisture content. Contact a local surveyor for help evaluating your hull and locating a lab. 

Special preparations for new boats 

An epoxy barrier coating is often applied to brand new hulls to avoid blister problems. New boats, obviously, have no blisters or water in the laminate, but they may require preparation not required by older boats. Check with your boat’s manufacturer to be sure this procedure does not negate the hull warranty. 

If bottom paint was applied by the boat manufacturer, it must be completely removed by scraping, sanding or chemical stripping before applying the barrier coat. Failure to do so will result in the epoxy bonding to the paint, rather than to the gelcoat. In such instances, the epoxy’s bond to the boat is only as good as the bond of the old paint to the hull. The gelcoat must be thoroughly sanded to a clean, dull surface. 

When new boats are delivered, provided bottom paint was not applied by the factory, they usually will have mold release agents or waxes on the gelcoat surface. These agents will prevent the epoxy coating from achieving a good bond to the gelcoat and must be removed. Sanding alone is usually ineffective since wax or silicone tends to clog the sandpaper, making it very difficult to remove all traces of the substances. 

1. Wipe the gelcoat surface twice, using a quality silicone and wax remover and clean, white paper towels. (The dyes used in patterned toweling may also contaminate the surface.) Wipe well above the waterline to minimize the chance of contaminating the surface while you’re working on the bottom. 

2. Abrade the gelcoat below the waterline. This can be done with 80-grit sandpaper on
an orbital sander, a double action (DA) sander or an air file, or by waterblasting or sandblasting. The entire hull surface must be dull with no shiny patches visible.