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.
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.
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.
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 undercured.
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. 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.