Repairing Gelcoat Cracks and Chips - SailNet Community

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Old 11-06-2002
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Repairing Gelcoat Cracks and Chips


Repairing cracks, scratches, and dings in gelcoat is a fairly straightforward matter, and it can make your boat look years younger.
Last month I detailed methods of restoring the shine to dull and faded gelcoat in Recovering the ShineUnfortunately, a loss of gloss is not the only damage fiberglass sustains. Everyone who has ever taken a boat to a dock knows that scratches are a risk. Gelcoat can also crack like plaster or craze like porcelain. In some cases it evens flakes off like bad paint. Fortunately, all of these conditions can be repaired.

Scratch Repair    The most basic repair to gelcoat is filling a scratch. If you can do this successfully, you are well armed to make all other gelcoat repairs. Scratches are repaired by filling them with fresh gelcoat. Except for color matching, the process is as straightforward as it sounds.

You cannot repair a scratch with gelcoat resin anymore than you could repair it with paint. To fill the scratch you need a thickened form of gelcoat, called paste. Scratch-repair kits containing a small amount of gelcoat paste and hardener along with a selection of pigments are widely available for less than $20.


When it comes to matching gelcoat colors, white is the easiest shade to work with.
Color Matching   
Even professionals who do gelcoat repairs daily can have difficulty getting a perfect color match. Condition yourself to be content with a self-assessment of "not bad." You can buy gelcoat paste in "factory" colors instead of using a kit that requires you to mix the color using supplied pigments—but if your boat has spent a few years in the sun, even factory colors won't match exactly. The solution is an adjusting kit— a selection of pigments. For small repairs to white boats, the generic kit is probably just as good. Color matching is much easier with white, and once the repair is buffed out to a gloss, small shading differences will be unnoticeable.

Matching colored hulls is more difficult. A house-paint color-sample card that closely matches your hull can provide valuable help. The paint-store custom mixes the color by adding tints to a white base, so ask the clerk for the formula. It may call for a half-dozen different tints, but the important ones are those specified in the largest quantities. Use the tints in your kit to approximate the formula.

Put exactly one ounce of paste into a mixing cup and add 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.

Color the gelcoat before you add the catalyst. As a general rule, four drops of hardener will catalyze 1 ounce of resin at 1 percent, but follow the instructions in the kit. Stir in the hardener thoroughly; if you fail to catalyze every bit of the resin, parts of the repair will be undercured. The mix shouldn't start to harden in less than 30 minutes. Always err on the side of too little hardener.


Because polyester resin shrinks when it cures, you should leave a little excess paste on your repair to ensure full coverage.
Applying Gelcoat    
While the original gelcoat is chemically bonded to the underlying laminates, the bond between the gelcoat paste and the long-cured hull is going to be strictly mechanical. To give the paste something to grip, sand wide scratches with 80-grit paper near the cracked or crazed area. Open narrow scratches into a shallow V by dragging the corner of a screwdriver blade through them. And remember to clean the scratch with an acetone-dampened rag before you attempt to fill it.

When you have the colored and catalyzed paste ready, wipe the scratch with styrene. This is to reactivate the old gelcoat, at least partially, which we hope will result in some chemical cross-linking. Now apply the paste like any other putty, using a plastic spreader. Because polyester resin shrinks slightly as it cures, let the paste bulge slightly. You are going to sand the patch fair anyway, but don't make the bulge excessive or you will create extra work for yourself.

Gelcoat will not fully cure in air, so you have to seal the repair with plastic. The kit may include sealing material. A section of a plastic kitchen bag also 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.

"Gelcoat will not fully cure in air, so you have to seal the repair with plastic."

Fairing    After 24 hours, peel away the plastic and sand the patch smooth using 120-grit paper wrapped around a section of 1 x 2 lumber. Use the narrow side of the block and short strokes to confine your sanding to the new gelcoat and never do this initial sanding without a block backing the paper.

When the patch is nearly flush, switch to 220-grit wet-or-dry paper. Using the block's wide side—and with a trickle of water running over the sanding area—feather the repair with a circular motion until your fingertips cannot detect a ridge. If the hull is curved, take care not to sand the repair flat.

Abandon the block and wet-sand the repair area with 400, then 600-grit paper until the surface has a uniform appearance. Then dry it and use rubbing compound to give the area a high gloss. Afterward finish with a coat of wax. If your color match is reasonably good, the repair will be undetectable.

Cracks    Last month I said that the thickness of gelcoat—typically about 10 times thicker than a paint finish—was both a blessing and a curse. Gelcoat owes its longevity to this thickness, and it is what allows us to rejuvenate gelcoat over and over by removing the top surface. But the thicker the gelcoat, the more brittle, so when the underlying laminate flexes for any reason, the gelcoat cracks.


Once you've finished the repair and waxed the surrounding area, it'll take a very close inspection to detect it.
Before you repair cracked gelcoat, you need to know why it cracked. If the cause is a lack of rigidity in the underlying laminate, your repair effort will be wasted unless you first reinforce the weak area. Additional laminates will stiffen the area, but adequate stiffness usually requires stringers or some type of core. Star-cracks radiating from beneath a stanchion base are common, and here the solution can be as simple as the installation of a generous backing plate.

Once you have corrected the cause of the crack, begin your gelcoat repair by opening each crack into a shallow V by dragging the corner of a scraper or a screwdriver through it. In effect, this changes the crack into a scratch, and from here on the repair is exactly like repairing a scratch. Never try to repair a crack without opening it. Gelcoat resin will penetrate the crack but it will not fill it, and gelcoat paste will simply bridge across the top of the crack.

By the way, if you are repairing a large area, you will want to seal it with polyvinyl alcohol (PVA) rather than sheet plastic. This is brushed or sprayed onto the surface of the gelcoat where it dries into a thin plastic film. It can be removed with water when the gelcoat has cured.

Crazing    Older fiberglass boats sometimes develop a condition known as crazing or alligatoring over large areas of the boat. In these situations the gelcoat becomes a riot of tiny cracks, looking not unlike the shell of a boiled egg when you crack it in preparation for peeling. If the crazing is localized—either side of the companionway, for example—flexing is the cause and no repair is possible until you stiffen the laminate. If, however, the crazing is widespread, the likely reason is the thickness of the gelcoat. Heat and cold cause the unreinforced gelcoat to expand and contract, and when it is too thick it eventually cracks like dried mud. A faulty resin formulation can also cause crazing.

"There is bad news and good news about crazing. "

There is bad news and good news about crazing. The bad news is that the only effective way to restore crazed gelcoat is to grind away most of it. This is very labor-intensive—both in the grinding and in getting the new gelcoat smooth. You will recall from my previous article that gelcoat has poor flow characteristics, so it must be sanded smooth, then buffed to a shine—a very big job for anything more than a small area.

The good news is crazing is almost never repaired with gelcoat. Instead, you prep the surface for painting, then coat it—particularly the crazed areas—with a high-build epoxy primer. The primer will penetrate and fill the tiny cracks, and the epoxy is strong enough to withstand future stresses on the gelcoat. It normally takes two, and sometimes three coats of primer. Sand the primer with 120-grit paper and apply two coats of a two-part polyurethane. The result is a crack-free, glassy surface without the application of sandpaper, compound, or polish.

Flaking    Flaking usually occurs because the manufacturer sprayed the gelcoat into the mold, but then did not immediately begin the laminate process. Consequently, the bond between the gelcoat and the laminate is weak. If the gelcoat cracks for any reason, pieces of the gelcoat fully circumscribed by cracks come loose and flake off. The only effective repair for flaking is to peel or grind off the loose gelcoat and replace it. If you intend to paint the repair, using thickened epoxy rather than gelcoat paste is recommended because the epoxy will have far superior adhesion.

That's about it. Repair a scratch first before deciding to—or not to—take on anything more. If I failed to answer all your questions, I again refer you to Sailboat Hull & Deck Repair, which covers fiberglass repair in great detail, with the added benefit of plentiful illustrations.


 Suggested Reading List

Recovering the Shine by Don Casey
New Shine for an Old Hull by Don Casey
Old Hull, New Coat by Tom Wood

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