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How To Repair a Fiberglass Boat

Few things are more disheartening to the boat owner than staring at the fuzzy edge of broken fiberglass. However, the repairability 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. And the repair is less patch than graft — a new piece of skin indistinguishable from the old.

Fiberglass lay-up is nothing more than layers of glass fabric saturated with polyester (or epoxy) resin, yet most boatowners 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

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, 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 sufficient 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 the full size of the ground depression, then cut subsequent pieces about an inch smaller. This order of largest piece first, then progressively smaller pieces is how you are going to apply the new fabric. We do it this counterintuitive way, particularly with polyester resin, because it maximizes the area of the secondary bond, the adhesion of the new cloth to the old laminate.

Using Polyester or Vinylester Resin

For above-the-waterline repairs you can use either polyester or vinylester 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 vinylester 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 vinylester 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 undercured, 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, 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.

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Don Casey

Contributor, BoatUS Magazine

Don Casey has been one of the most consulted experts on boat care and upgrades for 30 years, and is one of the BoatUS Magazine's panel of experts. He and his wife cruise aboard their 30-footer part of the year in the eastern Caribbean. His books include Don Casey's Complete Illustrated Sailboat Maintenance Manual, and the recently updated This Old Boat, the bible for do-it-yourself boaters.