For scratches in gelcoat up to 1/32 of an inch deep – Try sanding them out first with a hard rubber block using 320 grit water sand paper followed by 800 and 1000. Wash block, sand paper and job between grits. Don’t stay in one spot too long during sanding to avoid a dip, work the area as well. Keep a close eye to make sure you don’t sand through. Scratches in reds, blues, greens and so on may appear lite but could be fractured gelcoat through to the fiberglass from impact showing a white line from the stretch point and cannot be sanded out.
Repair Scratches In Gelcoat On Your Own
Boat Hull Scratches
Polishing compounds
Gelcoat Polishing
Polish gelcoat with 3m-super duty compound for first polish followed by meguiars diamond cut compound for a brilliant shine using an adjustable polisher with speeds up to 2400 rpms. Be sure to use a white wool polish pad for first cut then a finer yellow wool pad for final compound to finish. Be sure to wash off the super duty compound before applying the Diamond cut with the yellow pad, avoiding grit contamination.
If sanding gelcoat scratches on true colors as red, blue, green etc. Sand a small area first in a discreet area to be sure there is not a big change in color due to fading. Do not remove dry compound with polisher, running a dry pad will create wheel marks.
Once gelcoat is chaulked well a good polish will only last a month or so at most before clouding up again.
Course cut polishing pad
Finel cut
Deep Scratches
Simply roughing up the inside of a scratch and filling it with a gelcoat paste then a sand and polish will be quite visible around the edge of the scratch and will shrink during polishing , the same goes for chips and air voids. A gelcoat patch kit may be a quick fix but may be very noticeable on your boat hull. If you are looking for a professional boat repair, a fill and spray is the technique needed to achieve that as shown in this fiberglass and gelcoat repair video. Scratches in true color red, blue, green, black and so on are tough even for the professional and usually have to be matched and sprayed to a large area to blend using clear gelcoat for a blender. Gelcoat patch kits are a very noticeable fix on true colors. Some outfits match polyurethane paints like Imron for spot repair because it is easier to match and blend, the problem is the paint will eventually peel around the edges and eventually all the paint will peel off, avoid painting directly over gelcoat. Any paint products and clears will flake off maybe sooner than later if gelcoat is not primed first. Gelcoat will stick to gelcoat best for spot repairs.
                      Repair Boat Hull Scratches