Whether you’re a marine hobbyist or a professional, there’s a lot to know when it comes to building a boat and maintaining it. Below, you’ll find an ever growing library of do it yourself information.

Current Topics

A Primer on Resins

Fiberglass Blister Repair

Fiberglass Repair

Gelcoat Scratch Repair

A Primer on Resins

There are three main types of laminating resin systems used in the marine composites industry; polyester based, vinyl ester based and epoxy based. Each has their own strengths and weaknesses so it is necessary to use the resin that is right for your application.

Polyester Resin

How it Works: Combines with the catalyst Methyl Ethyl Keytone Peroxide (MEKP) to create a hardened product.

Best Use: Repairing any polyester product including most fiberglass boats.

Application Advice: The least expensive of the three types of resins, Polyester resin is generally easy to use but not necessarily good for bonding to vinyl ester or epoxy laminates. It’s most suitable for above waterline repairs because it tends to let water pass through more easily than epoxy or vinyl ester, so it’s more prone to blistering.

Epoxy Resin

How it Works: Cures basically in the same manner as two-part polyester resin, but uses different chemical bases that provide better adhesion, strength, hardness, durability and water resistance. Epoxies are mixed by ratio rather than percentage like polyester and vinlyester resin.

Best Use: Wood, plastic and fibreglass repairs above or below the waterline and as a barrier coat.

Application Advice: Epoxy won’t blister like polyester because it absorbs less water and is known for excellent properties, such as chemical/heat resistance and ability to bond to other materials. However, if your original repair was made with epoxy, then it’s best to stay with epoxy. Epoxy is more expensive and tends to take longer to cure than polyester or vinyl ester- generally 24 hours between coats. It is most often used for cedar strip canoes, sports equipment etc. When used in laminating epoxy is usually used with boat cloth or fabrics such as carbon fiber, Kevlar, boat cloth.

Fiberglass Blister Repair

Fiberglass blisters occur because water passes through the gelcoat. Water soluble chemicals inside the laminate exert an osmotic pull on water outside, and some water molecules find a way through the gelcoat. As more water is attracted into the enclosed space, internal pressure builds. The water molecules aren't squirted back out the way they came in because they have combined with the attracting chemicals into a solution with a larger molecular structure. Instead, the pressure pushes the covering gelcoat into a dome-a blister.

There has been a great deal of hysteria about blisters, but the reality is that the number of boats that develop serious blister problems is extremely small. An occasional blister or two is not a serious problem, any more than is an occasional gouge in the hull. Some boats seem to exhibit a greater propensity to blister, presumably due to the chemical components used and/or the layup schedule, but all boats are at some risk. Surveys suggest that about one boat in four develops blisters.

Repair materials

Effecting the repair of a few hull blisters requires an appropriate quantity of epoxy resin and hardener. Do not use polyester resin for blister repairs; you need the stronger adhesion and better water impermeability epoxy provides.

You also need a filler to thicken the epoxy into a putty. Select colloidal silica. Never use microballoons or any other hollow or absorbent (talc, for example) fairing compound to fill blisters.

A quart of acetone, a box of TSP (trisodium phosphate), a few acid brushes, and a 36-grit sanding disk completes your supply list. If the blisters penetrate the laminate, you may also need a yard of 6 to 10 ounce fiberglass cloth.

Minor Blistering

The first step in minor-blister repair is opening the blister to let it drain. Pop the dome with a chisel, screwdriver, or rotary tool. Be sure you are wearing eye protection; pressure inside a hull blister can be double that of a bottle of champagne, and the fluid that blasts out when you pop it is acid.

Load a disk grinder with your 36-grit disk and grind the open blister into a shallow depression. The rule of thumb is that the depression should be 20 times as wide as it is deep, and it should only be as deep as required to remove any damaged laminate beneath the gelcoat.

Use a plastic mallet or the handle of a screwdriver to tap the hull all around the blister. Sound laminate will give a sharp report. A dull or flat sound anywhere indicates additional delaminating, meaning that the blister is larger than you thought. Increase the circumference (not the depth) of the depression until the laminate all around it is sound.

Flush the open blister with water, and then scrub it squeaky clean with a solution of hot water (if available) and TSP-about a quarter cup of TSP to a gallon of water. Rinse thoroughly, and then allow the blister to dry for at least 48 hours, longer if practical. If you dry-store your boat for the winter, grind and scrub blisters at haul out but don't fill them until launch time.


Just before filling, scrub each depression briskly with a clean rag dampened with acetone.

Mix a small quantity of epoxy (one pump) and paint this unthickened resin into the cavity. Wet out the entire surface of the depression. Use an acid brush to apply the epoxy and give this application 20 or 30 minutes to begin to kick. For shallow blisters, prepare a small amount of fresh epoxy (one or two pumps) and thicken it to peanut butter consistency with colloidal silica. Fill the depression completely with this mix, using a squeegee to compress and fair the filler. Silica-thickened epoxy is difficult to sand, so take extra time to fair the epoxy as well as possible while it is wet.

Deep blisters require the replacement of the damaged glass fabric. Cut a disk of fiberglass cloth the size of the bottom of the depression, then cut several more, each a little larger than the last. Use only cloth; never use fiberglass mat with epoxy resin.

Wet the bottom of the cavity with epoxy and lay in the smallest disk of cloth. Wet out the cloth with resin until it is transparent, and then lay in the next, slightly larger disk. Wet this one out, using the end of the brush to tamp the disks and compress them together. Continue adding disks and saturating them with resin until the repair is even with the surrounding surface.

Whether you have filled the cavity with glass disks or epoxy putty, allow the filler to kick. When the epoxy is no longer fluid, but still tacky, paint the repair and an inch or so beyond with at least two coats of unthickened epoxy, letting each coat kick before applying the next.

Let the repair cure for 24 hours, then scrub it with water and an abrasive pad (like Scotchbrite) to remove the waxy film on the surface of the epoxy. Fair the repair with a sanding block and you are finished.

Gelcoat should never be applied over epoxy. Since the repair will be covered with bottom paint, there is no need for a gelcoat surface anyway. Don't use gelcoat in blister repair.

Boat Pox

Boat pox is a much more serious condition, related to the occasional blister like acne to the occasional pimple. If the bottom of your boat is covered with blisters, filling them won't cure the problem. Pox is a systemic condition indicating that the hull is saturated. The actions necessary to remedy boat pox require specialized equipment and expertise.

Fiberglass Repair

Burnside Fiberglass Marine Supply recommends that for any non-cosmetic repairs below the waterline or for structural repairs you should contact a professional repair facility for advice.

Few things are more disheartening to the boat owner than staring at the fuzzy edge of broken fiberglass. However, the repair ability of fiberglass is one of its best characteristics. The most horrifying hole in a fiberglass hull is quickly healed with a bit of glass fabric, a supply of resin, and equal parts skill and care.

Fiberglass lay-up is nothing more than layers of glass fabric saturated with polyester (or epoxy) resin, yet most boat owners imagine a self-applied repair as only slightly more durable than a wet Band-Aid. This is a false concern. Follow a few simple rules and your lay-up will be just as durable as the rest of the boat.

Cut Away the Damage

Impact damage nearly always results in some associated delamination. Tap the impact area with the end of a plastic screwdriver handle to determine the extent of the damage; solid laminate sounds sharp, delamination dull. Check inside the boat to make sure nothing is in the way, then make a circular or oval cut to remove the damaged area. Never try to save damaged fiberglass; always cut it out and replace it with new laminate. Check all the edges and enlarge the hole if you find any additional delamination.

Working from Inside

If the damage area is small and above the waterline, make the repair from inside the hull, if possible. You are going to bevel the edge of the hole with a 12-to-1 chamfer, so if you repair a 3-inch diameter hole through a 1/2-inch-thick hull from the outside, you end up with about 15 inches (diameter) of surface damage to refinish. Repair it from the inside and you have only a 3-inch hole to refinish.

A second reason to make the repair from the inside is that you can back the hole on the outside with a polished surface, creating a mold that allows you to lay-up the repair the same way the boat was built-gelcoat first. Very little finish work will be required.


Before grinding; always wash the area around the hole thoroughly with a dewaxing solvent. The original fiberglass will have traces of mold release on the outer surface and wax surfactant on the inner surface. If you fail to remove the wax first, grinding will drag it into the bottom of the scratches and weaken the bond.


During the lay-up process, because each layer is applied before the previous one fully cures, each application of resin links chemically with the previous one to form a solid structure-as though all the layers were saturated at once. Unfortunately, no matter how strong the laminate-to-laminate bond, the initial bond of any repair is mechanical, not chemical. Consequently, grinding is the key to getting a strong repair.

Use a disk sander loaded with a 36-grit disk to grind a 12-to-1 bevel around the perimeter of the hole inside. Also grind a rectangular area of the inner surface a few inches beyond the bevel to accommodate a finishing layer of cloth. Protect your eyes with goggles and your lungs with a good dust mask. Long sleeves will reduce skin irritation. Tilt the sander so that only one side of the disk is touching the surface and the dust is thrown away from you. After you brush away the dust and wipe the area with an acetone-dampened rag, the sanded surface should have a uniform dull look.

Mask and Mold

To prevent any resin runs from adhering; give the exterior surface of the skin around the hole a heavy coat of paste wax, taking care not to get any on the edge or inside the hole. Mask the area below the hole.

Cut a scrap of smooth plastic laminate (Formica) or thin clear acrylic (Plexiglas) a foot larger than the hole. Wax this backer, and then spritz it with polyvinyl alcohol (PVA) mold release. Screw or tape the backer to the outer surface. If the hull is flat or curving in only one direction in the damage area, the backer will assume the correct curve. If the hull is spherical, i.e. curving in two directions, acrylic screwed to the hull can sometimes be coaxed into the correct shape if warmed with a heat gun (before applying the mold release).


Cut the fiberglass fabric to fit the hole. Unless you have reason to follow a different schedule, begin with two layers of 1 1/2-ounce mat, then alternate mat and 6-ounce cloth. The number of laminates will be determined by the thickness of the hull; you will roughly need one layer for every 1/32 inch. Cut the first layer of mat 1 inch larger than the hole, i.e., overlapping the bevel by 1/2 inch all around. Subsequent pieces should be 1/2 inch larger all around than the previous one.

Using Polyester or Vinylester Resin

For above-the-waterline repairs you can use either polyester or vinyl ester resin. Of course, for an even stronger repair you can also use epoxy, but not if the surface of the repair will be gelcoat. (You should use epoxy for underwater repairs.)

If you are doing your repair with polyester or vinyl ester resin, you need laminating resin. Laminating resin does not fully cure while exposed to air, which allows you to get a chemical bond between the multiple laminates you will be applying. To get the final laminate to cure, you simply seal it from the air, either with a plastic or by coating it with polyvinyl alcohol (PVA) mold release.


The catalyst for both polyester and vinyl ester resin is methyl ethyl ketone peroxide, or MEKP. Do not confuse MEKP with the common solvent MEK; they are not the same.

Polyester resin usually requires 1 to 2 percent of hardener by volume (follow the manufacturer's instructions). As a rule of thumb, four drops of hardener will catalyze 1 ounce of resin at 1 percent. Be certain to stir the catalyst in thoroughly or part of the resin will be under cured, weakening the lay-up.

You can adjust the cure time by adding more or less catalyst. Temperature, weather, and the thickness of the laminate all affect curing times. Some experimentation is generally required. The mix shouldn't kick (start to harden) in less than 30 minutes. Hardening in about two hours is probably ideal, but overnight is just as good unless the wait will hold you up. Always err on the side of too little catalyst; if you add too much, the resin will "cook," resulting in a weak lamination.


Gelcoat is essentially pigmented polyester resin. Start the repair by spraying or brushing about 20 mils of color-matched gelcoat onto the waxed backer. Check the gelcoat thickness with a toothpick; 1/32 is about 30 mils.


When the gelcoat kicks, wet it with polyester resin and lay-up the first two layers of mat and one layer of cloth, compressing them against the gelcoat and working out all voids and bubbles with a resin roller and/or a squeegee.

Let the first three plies kick, and then lay up four additional plies. Never lay up more than four plies at a time or the generated heat may "cook" the resin and weaken it. Continue the lay-up four plies at a time until the repair is flush with the interior surface.


For a finished look, cut a rectangular piece of mat and one slightly larger of cloth and apply these over the patch, smoothing them with a squeegee. Seal this top layer with plastic or PVA to allow a full cure.

Remove the backer from the exterior surface. Fill imperfections in the new gelcoat with gelcoat paste and allow it to cure fully. Clean the area around the patch, then sand-if necessary-and polish the repair area.

Gelcoat Scratch Repair

Surface scratches can be buffed out of gelcoat with polishing compound, but deep scratches must be filled. When the gelcoat surrounding a scratch is in good condition, the filler of choice is gelcoat paste, which provides both filler and finish in a single application-but not a single step. Because the surface of the cured paste will be uneven, sanding and polishing are required to smooth the repair and blend it with the rest of the hull. Except for color matching, gelcoat repairs are easy and straightforward.

Gelcoat Choices

You will find gelcoat available as both a resin and in a thicker putty form called paste. For scratch repair you want paste. Repair kits comprised of a small amount of gelcoat paste and hardener, a selection of pigments, mixing sticks, and sealing film can be purchased for less than $20. Buy a flexible plastic spreader if you don't already have one. You will also need sheets of 150-, 220-, 400-, and 600-grit wet-or-dry sandpaper. A single sheet of each will be more than ample to fair all the paste in a repair kit.

If you are repairing several scratches, add a small bottle of styrene to your list of supplies. Wiping each scratch with styrene just prior to filling it partially reactivates the old gelcoat, resulting in some chemical bonding between the old gelcoat and the new. Otherwise the bond between old and new is strictly mechanical-like a coat of paint. A mechanical bond is normally adequate, but the more extensive your repair, the more certainty you want.

Color Matching

The hardest part of a repair to the surface of a fiberglass boat is matching the color. Professionals who do gelcoat repairs daily still have difficulty getting a perfect match. Even "factory" colors don't match exactly after a boat has been in the sun for a few years.

White has the significant advantage of being fairly easy to match, and once a small repair is buffed out to a gloss, shading differences will be unnoticeable. Matching colored hulls is somewhat more challenging.

A color-sample card from your local paint store that matches your hull can provide valuable help. Ask the store clerk the formula; they custom-mix the color by adding tints to a white base. The formula may call for a half-dozen different tints, but the important ones are those specified in the largest quantities. You can use the tints in your repair kit to approximate the formula.

Always color gelcoat paste before you add the catalyst. Put exactly one ounce of paste into a mixing cup and add the tints a drop at a time. Keep track of the number of drops of each tint. When the color looks close in the cup, touch a drop of the mix onto the hull. Make needed adjustments until you are satisfied with the match-don't expect perfection-then write down the formula so you can duplicate it for the rest of the paste.

Preparing the Scratch

Never try to repair a scratch by simply painting over it with gelcoat. Gelcoat resin is too thin to fill a scratch and gelcoat paste is too thick. Instead of penetrating scratches, gelcoat paste will bridge them, leaving a void in the repair. To get a permanent repair, draw the corner of a scraper or screwdriver down the scratch to open it into a wide “V”. This is the time to wipe the open scratch with styrene to reactivate the old gelcoat.


The hardener for gelcoat is the same as for any polyester resin-methyl ethyl ketone peroxide, or MEKP. Gelcoat resin usually requires 1 to 2 percent of hardener by volume (follow the manufacturer's instructions). As a general rule, four drops of hardener will catalyze 1 ounce of resin at 1 percent. The mix shouldn't kick (start to harden) in less than 30 minutes. Hardening in about two hours is probably ideal. Always err on the side of too little hardener. Also be certain to stir in the hardener thoroughly; if you fail to catalyze every bit of the resin, parts of the repair will be under cured.

Spreading Gelcoat Paste

Work the gelcoat paste into the scratch with a flexible plastic spreader. Let the putty bulge a little behind the spreader; polyester resin shrinks slightly as it cures, and you're going to sand the patch anyway. Just don't let it bulge too much or you'll make extra work for yourself.

Scrape up any excess paste beyond the patch area.

Covering the Repair

Gelcoat will not fully cure in air. To seal the surface of a scratch repair, cover it with a sheet of plastic film. The kit may include sealing film. Otherwise a section of kitchen "zipper" bag works especially well because it tends to remain smooth and the gelcoat will not adhere to it. Tape one edge of the plastic to the surface just beyond the repair, then smooth the plastic onto the gelcoat and tape down the remaining sides.

Sanding and Polishing

After 24 hours peel away the plastic. The amount of sanding required will depend on how smoothly you applied the gelcoat.

A 5 1/2-inch length of 1 x 2 makes a convenient sanding block for a scratch repair. Wrap the block with a quarter sheet of 150-grit paper, and use the edge of the block to confine your sanding to the new gelcoat. Use short strokes, taking care that the paper is sanding only the patch and not the surrounding surface. Never do this initial sanding without a block backing the paper.

When the new gelcoat is flush, put 220-grit wet-or-dry paper on your block and wet sand the repair, this time with your block flat. Use a circular motion and keep a trickle of water running on the sanding area. Feather the repair into the old gelcoat until your fingertips cannot detect a ridge. If the hull is curved, take care not to sand the repair flat.

Abandon the block and switch to 400-grit wet-or-dry paper. Wet sand the surface until the repair area has a uniform appearance. Follow this with 600-grit wet-or dry. Wear cloth garden gloves-the kind with the hard dots-to save the tips of your fingers.

Dry the area and use rubbing compound to give the gelcoat a high gloss. Swirl a soft, folded cloth over the surface of the compound to load the cloth, then rub the compound onto the repair area. Buff it with a circular motion, using heavy pressure initially, then progressively reduce the pressure until the surface becomes glassy. If the gelcoat shows swirl marks, buff them out with a very fine finishing compound.

Finish the job by giving the repair area a fresh coat of wax. If your color match is reasonably good, the repair will be virtually undetectable.