When is it appropriate to sand out or buff out gelcoat damage and when to actually use a gelcoat repair kit, along with visual instructions to fix the damage.

By Paul Mirto, Capt. Patrick Sciacca

Always isolate the damaged area with masking tape first

I remember the scene vividly. The just-painted battlewagon was being brought into her summer slip. As the captain expertly backed the vessel, a burst of wind suddenly threw her bow to port. At least there was a dock wheel to catch the aft starboard corner. Otherwise that brand new shiny hull side would’ve laid up against the knarly slip corner. Unfortunately, the dock wheel collapsed and the corner gouged a deep four-foot-long scar down the hull side. Flakes of yellow paint and fiberglass wafted on the breeze and across the canal like scattered ashes.

1. Grind out an area slightly beyond the scratch. A Dremel tool is effective.

2. Filler shrinks while drying, so be sure to put in a little extra.

It wasn’t pretty, but these things happen. And if your boat has ever been the victim of an asleep-on-the-job dock wheel, a mussel-encrusted piling, stress cracks, or an anchor ding, there is hope. Whether the damage is pinhole-size or deeper, it can be fixed—maybe by you.

But before it’s time to take action, you need to determine the extent of the injury. With a scratch or abrasion, a less-is-more approach may best serve you.

3. A block sander prevents you from gouging the repair.

4. Spraying on gelcoat helps ensure its thickness and finish.

According to Jim Christenson, a marine sales representative for 3M, which makes a host of repair products, you should start with the least-aggressive method to repair the gelcoat, which is essentially a resin-based liquid that’s sprayed into a hull mold prior to lay-up. Minor imperfections can usually be wet-sanded and buffed out, says Christenson, adding that sometimes a rubbing compound may also be used. The wet-sanding process usually involves 1000-grit, and perhaps even 2000-grit, sandpaper to reawaken the gelcoat’s original gloss. Christenson also recommends using a slow-moving random orbital sander in constant motion in all directions. The slow movement prevents you from gouging the gelcoat and, in turn, the fiberglass beneath. If you’re hand sanding, use a block to prevent digging into one area.

When the damage is deeper or sanding alone is not working, you may need to employ a buffing wheel and compound, which companies like 3M, Evercoat, Minicraft, and Spectrum all offer.

Prior to applying the compound to the scratch or abrasion, wash the area with soap and water to prevent the compound from getting contaminated and causing even more scratches. Next, break out that buffing wheel. According to 3M, the buffing pad should be wool for better cutting and the polishing wheel pad a blend of synthetics and wool. Whichever you’re using, make sure to keep it flat to the surface to prevent the wheel from digging in and reduce pressure with each pass of the wheel. Do not run the wheel faster than 2000 to 2400 rpm, says Christenson. It is possible to over-buff an area and wear away the compound or polish, ultimately degrading the gloss you’ve just created. Once the area is scratch-free, you can finish off the repair with polish and that wheel again. Use a lighter wool or wool-blended pad for the final polishing step.

When the damage is beyond the abilities of a wheel and elbow grease, you can purchase a gelcoat repair kit at many marine-supply stores. Prices generally range from $12 to $50.

Some have a premixed filler and gelcoat with tinting agents, and with these, the toughest part can be matching the gelcoat color to that of your boat’s hull, especially accounting for oxidation. Frank Herman, who has three decades in the boatbuilding and repair business and owns Mobile Fiberglass Repair in New York, says that matching gelcoat color is a true art. “If I have an older gelcoat and it’s oxidized, I’ll polish and wet-sand [the damaged area] .with 2000-grit paper. Then I’ll compound and polish it to get as close to the original color as I can. That’s what I’ll do my color-match from; it’s a truer match,” he adds. Over time, the new gelcoat will fade so that it matches the rest of the boat.

But when do you know if it’s a repair that you can handle or if it’s time to call someone like Herman?

The answer is partially determined by your confidence and DIY initiative and partially by the size of the job—it may be too big or perhaps the damage indicates that there is a structural issue. Regarding the latter, stress cracks, which often appear to be only cosmetic, can suggest a bigger problem beneath. Herman explains that many stress cracks occur because of an inaccuracy in the hull-to-deck joint during construction. He explains, “Once the boat is running and the deck is under stress, all the corners settle and start to work.” Mat and roving are more flexible than gelcoat, and the result is stress cracking.

The Painting Solution If a spot fix won’t do on your boat’s gelcoat problem, perhaps it’s time to paint. Interlux makes epoxy fillers, resins, and of course, one- and two-part polyurethane paints that can fix that gelcoat damage and make your vessel’s hull sides shine just like new. The company’s Web site provides a plethora of tips on everything from choosing the right product for your application to how to apply it to your boat. In short, Interlux recommends cleaning and dewaxing the hull to remove any contaminants. Be sure to fill in any scratches and dings with epoxy filler and then sand it smooth. Sand the hull to a dull matte finish, which will provide a good anchor patter for the primer and remove any of the old oxidized gelcoat. Next, apply two coats of primer paint, sanding thoroughly to a smooth finish. Lastly, apply two coats of a quality marine

Sometimes the cracks are created in the hull mold. During lamination the gelcoat spray gun can pass over one area of the mold several times more frequently than another, which thickens the gelcoat in that spot. Once the area flexes, the thicker part can begin to crack.

In an effort to deal with stress cracks, some DIY’ers will dig out the dings with a Dremel tool, fill it with putty, and spray it. (Always grind out an area slightly beyond the damaged section to ensure a solid bond.) But Herman warns that many times the stress crack will reappear and suggests that there’s an even better way to prevent future cracking problems.

He believes that the fiberglass needs to be ground out and new fiberglass laid in a little heavier than the original layup in order to make the area stronger. Once cured, the new fiberglass is filled and faired before being sprayed with a thin layer of gecloat that has more flex. Herman says to further ensure the cracks won’t come back he likes get behind them and add fiberglass there, too. He also suggests making these types of repairs only when the air temperature is between 48ºF and 85ºF.

So now you know that there are myriad ways that you (or a guy like Herman) can fix that scratch. But the best way is to avoid them to begin with. So keep those fenders at the ready and beware of errant winds—and those faulty dock wheels, too.

Mobile Fiberglass Repair (632) 664-1994.

3M (877) 366-2746.

Minicraft (800) 282-8244.

Spectrum (800) 754-5516.

Evercoat (513) 489-7600.