How To: Repair Fiberglass

Fiberglass Repair: DIY (or Not)

Stress cracks, dock dings and other minor injuries aren’t difficult to repair yourself. Anything bigger?
Here’s some info to help you decide when to call in a pro.

Unless your boat lives her life dockside cuddled up to a couple of fenders, she’s going to get a few scrapes and bruises. And almost every fiberglass boat, even the most mollycoddled, eventually suffers the heartbreak of spider-web and/or stress cracks. These scars of a life well-lived are unsightly, but usually are just cosmetic. Generally, repair of minor damage is within the mechanical aptitude of most skippers. DIY’ing these small, but time-consuming, jobs will save big bucks at the boatyard, too.

But what about bigger, more serious injuries? I strongly advise against DIY if there’s a hole punched through the side of your boat, a deep impact crater revealing torn fibers or, even worse, coring, or any serious damage to the laminate itself. These are, I’d say, jobs for the pros. Take the time, fork over the cash and have it done right. My rule of thumb is, if I have to buy new tools to do the job, I don’t. I suck it up and pay the money.

First, Do No Harm

Stress cracks and crazing themselves aren’t usually structurally damaging, only unsightly; left untreated however, they can eventually let water seep into the laminate and maybe do real damage over the years. Ditto for minor dings and gouges. If the underlying laminate isn’t damaged—showing broken strands of fabric or cracked resin, for instance—many skippers just leave things alone. But are you sure the cracks and crazing are only cosmetic?

M. Boyd Siegel is assistant crafts manager at Saunders Yachtworks in Gulf Shores, Alabama, and he’s also aces at fixing fiberglass laminates. Sometimes it’s hard to tell how serious the damage is, Siegel says. “Gelcoat failure often shows as single small cracks or crazing, erratic cracks that are rather shallow. Lamination failure will show concentric fractures that radiate outward from a central point of impact.” Both issues can look similar, and the only way to be sure is to open up the damaged area and look at the base laminate.

“Opening up” usually means grinding with 60- or 80-grit abrasive disks. In skilled hands, an angle grinder makes short work of removing damaged gelcoat and fiberglass, just enough and no more. That’s where many DIY’ers run afoul. They take off too much, and anything that comes off has to be put back on, turning a small job into a big one. The rule of thumb is to grind out a crater around the damage with a 12:1 slope, creating an ample bonding area for the replacement fabric. For example, an impact gouge 1/4-inch deep would mean a 30-inch radius, or a crater 6 inches across. Grinding to a 3/8-inch depth increases the diameter to 9 inches, with 2.25 times the area. That means more than twice as much fabric will be needed to fill the crater, twice as much resin, twice as much squeegee-ing, twice as much gelcoat, twice as much fairing, sanding and buffing. Get the idea?

Unless you have a close relationship with your angle grinder, I’d hire a fiberglass pro from the yard to do the grinding, if not the whole job; he’ll also have his own tools and safety gear, which you probably don’t. (Wear a respirator, eye protection, long sleeves and gloves when working with fiberglass, by the way. The grinding dust won’t do your lungs any favors, and the acetone, styrene and other solvents aren’t so great, either. A simple dust mask isn’t enough.)

Experts recommend using a Dremel or Dremel-style rotary grinder to open stress cracks prior to filling. While easier to handle than a honkin’ big angle grinder, a Dremel can cut through eggshell-thin gelcoat and go too deeply into the laminate underneath if not wielded gently. If you have lots of cracks, get friendly with the Dremel. You can buy one that’s fine for gelcoat work for under $100. To fix isolated stress cracks, however, I’d take the advice of the folks at West System and use a simple can opener and sandpaper to widen the cracks. While you might make the same mistakes with hand tools as you would with power, you won’t make them as fast.

What About Resin?

Shallow gelcoat cracks with undamaged laminate underneath can be filled with gelcoat alone. Brush it on, or dribble it into the crack with the same popsickle stick you used to mix in the catalyst. But most repairs will involve resin—polyester, vinylester or epoxy. It wasn’t long ago that the only choice for most of us was polyester; that’s what marine stores carried. And polyester worked pretty well. Now it’s easy to buy any resin. But which one is best?

Traditional wisdom says to match repair resin with the original (i.e., whatever the boat was built with), but in this case tradition is behind the times. Most boats are still laid-up with ortho- or isophthalic polyester, but frequently there’s a layer of vinylester, a hybrid of polyester and epoxy, right behind the gelcoat to improve osmosis resistance. Maybe it gets ground off during the repair, or maybe not. And some higher-end boats are built entirely of vinylester. Vinylester resin sticks fine to polyester, so maybe that’s the choice? Nope—it’s easier than that. One resin fits all, and that’s epoxy. It’ll stick to almost anything, and has other advantages, too.

“While vinylester and polyester both have their places in vessel repair, any chance I get I will use epoxy,” says Siegel. “Epoxy may be more expensive, but it’s a far superior resin.” Repairs don’t use mass quantities of resin, so the extra cost will more than pay off in a higher-quality job, he adds. Epoxy is also more user-friendly, especially for the DIY’er. It can be mixed with slow-cure activators to extend working time, and fillers to make a thick adhesive putty or an easy-to-sand fairing compound. “Using the proper fillers, glass and activators can really help an inexperienced do-it-yourselfer turn out a nice, production-strength repair,” says Siegel.

When you surf the web for information on fiberglass repair, you’ll surely come across certain “experts” warning that polyester gelcoat won’t stick to epoxy. Not true, according to the folks at West. As long as the surface is properly prepared, gelcoat will stick fine to epoxy. But you must first remove its “amine blush,” a waxy compound that forms on the surface of the epoxy during cure. It scrubs off easily with water and an abrasive pad. Polyester resin should be cleaned the same way after cure, to remove wax that floats to the surface to seal out air and allow full cure. Laminating resin doesn’t have wax, so it stays tacky, an advantage when building up layers of fabric. If repairing fiberglass with polyester resin, be sure to buy it with wax, or else you’ll have to seal it somehow to make it cure all the way; usually, “cures tack-free” will be somewhere on the label. If you used the wrong resin and need to seal it, plastic wrap works, or you can spray it with polyvinyl alcohol (PVA). It’s easy to remove once the resin has cured. Same thing with gelcoat: It comes with or without wax. For repairs, you want it waxed.

An Eye for Color

The Achilles’ heel of many repairs is matching new gelcoat to old. With luck you can buy the correct color from the boatbuilder or an aftermarket supplier; otherwise you’ll have to get a set of pigments and tint the gelcoat yourself. I have no eye for color, so for me this means lots of trial and error. Siegel advises making it easier by first cleaning and polishing the area you’re trying to match, removing any grime or oxidation; then add small amounts of tint to the gelcoat. Keep track of how many drops of this color and that color you’re adding, so you can mix it again for the next repair job. Go slow—a little pigment goes a long way.

When you’re close to a match, daub a thin layer of un-catalyzed gelcoat on the existing surface and let it dry. “Gelcoat color will change slightly as the solvents evaporate; it typically tends to get slightly darker,” says Siegel. A sunny day makes this happen faster, and the un-catalyzed gelcoat will wipe off with solvent. Once you have a match, catalyze the gelcoat and apply it to the repair. Pros will spray it on, but DIY’ers usually brush; there are brushable gelcoats on the market that can be used right out of the can, without thinners or additives. Either method involves plenty of wet-sanding and buffing to get that new-boat shine.

Man, is this stuff complicated or what? Isn’t there an easier way, short of doing nothing at all? Siegel concludes, “There are always easy fixes, but they typically come at a cost. How long will the repair last? One day or twenty years? It all depends. The best way is to grind and start fresh.”

If removing old, damaged material and replacing it with new is the only way to ensure you don’t have to repair the same place twice, maybe it’s better to tap a professional and just pay once.

This article originally appeared in the September 2018 issue of Power & Motoryacht magazine.