The next time you're cleaning and waxing your boat, take the time to look for small cracks, scratches and chips in its gel coat. If your boat is more than a few years old, you'll probably be surprised at the number you find.
Most scratches and chips in gel coat result from impacts with hard objects (winch handles, downrigger weights, 15 lb. lobsters) and are not cause for concern. But if you find a series of cracks, take a minute to inspect the area more closely. If the cracks radiate from the base of load-bearing equipment like a cleat or stanchion, there is probably a problem with the installation that deserves attention before repairing the gel coat. Solving it might be as simple as shifting a load from undersized equipment, or installing a larger backing plate to spread the load over a wider area. If cracks appear at important joints or intersections in the cabin or deck, however, they might be the sign of an underlying structural weakness that needs to be examined. You might consider hiring a marine surveyor or having a qualified boat maintenance worker take a look at the problem to ensure that it isn't serious.
Before you begin, wash the area with soap and water and rinse it thoroughly. If the surface is oxidized, restore it with a rubbing compound so you'll be able to match its color accurately. Once the surface is clean and dry, mark off the repair area with masking tape.
Next, gouge out small, narrow cracks (and scratches that are too deep to remove with rubbing compound) until they are wide enough to fill with gel coat paste. A miniature grinding tool like a Dremel is ideal, but the sharp point of a can opener will work, too. (If you don't open the crack, you won't be able to force the gel coat into the repair area or expose enough surface area for the repair to adhere.) Then sand lightly with 220-grit sandpaper. After sanding, thoroughly clean the area with acetone to remove the sanding residue and any waxes or other contaminants that might interfere with the bond between the damaged surface and gel coat. Be sure to provide adequate ventilation and proper protection for your skin and eyes whenever you work with acetone.
The next step is to match the color of your existing gel coat. Start with a white or neutral gel coat paste (not resin) and begin adding tiny amounts of coloring agent. Mix several test batches of gel coat and pigment, add hardener and allow them to cure (gel coat changes color during the curing process). Once you've found an acceptable match (an exact one is nearly impossible), mix a final batch using the same ratio.
Next, using a putty knife, fill the areas to be repaired with the paste you've mixed. Force out any air holes and be sure to overfill, as gel coat has a tendency to shrink as it cures. When you're finished filling, seal the repair off from the air with a PVA curing agent or a piece of plastic wrap or wax papergel coat does not cure properly when exposed to air.
Once the gel coat has fully cured, sand the repair smooth (wet sanding works particularly well with gel coat). You can start with 220-grit sandpaper and, for a really slick surface, finish with at least 400- or 600-grit. Finally, apply a coat of high-quality marine polish and your repair is complete.