When looking at a scratch in the gelcoat, you need to determine whether it is a surface scratch or a deep scratch. The reason being is that surface scratches can easily be buffed with a rubbing compound as opposed to deep scratches which take a couple more steps. As long as the gelcoat around the deep scratch is in good condition, you will be able to use gelcoat paste or you can make one using gelcoat and an aerosil filler/thickener. Once the filler is applied, the service will be uneven so sanding and polishing will be necessary. Customers report wet sanding with 2000 grit worked well. Everything about the repair of the scratch is fairly easy to do.
For deeper gouges, you will need to purchase a gelcoat repair kit (see instructions here) or purchase gelcoat in various colors. Tints are available as well if custom matching larger quantities of gelcoat or paste. The kit will contain everything you need to do the job including some tints for matching. Some find the gouge needs to be scraped deeper with a screwdriver or Dremel tool to open it into a wide V to make sure the gelcoat has a place to settle and more permanent. If you simply cover the cracks they may continue to spread. The gelcoat that exists on the boat is already bonded with the laminates that are underneath. That being said, you will have to rough the surface up a bit with sand paper in order to give the paste something to stick to. Please note that you need to clean the surface with acetone after sanding to clean any contaminates off that may interfere with the filling process.
When the damage is a larger (like a screw hole) you made need 3M’s Preminum Marine Filler or 3M Marine High Strength Repair Filler. These are for bonding and filling repairs above or below the waterline and can be sanded. Some customers perfer to mix some thickener (cabosil) and gelcoat to a thick consistancy. Both of these products can be applied with a spreader. If coating over with gelcoat be sure to leave space (about 1/8 inch) for the gelcoat.
Once you have the gelcoat mixed, saving the hardener for last, begin to spread over the gouge with a plastic spreader. Since the paste will shrink slightly as it dries, make sure you put a little extra paste on the “wound”. A slight bulge will be ideal after the paste is applied. As the paste will not cure completely in the air, you will need to put a plastic film over it. If your kit does not have one, shrink wrap or wax paper will work. Tape the plastic/wax paper on all sides over the paste and let dry for 24 hours.
If the repair is larger that the 4oz repair
After the 24 hours are up, remove the plastic/wax paper. Take a piece of 400 grit sandpaper accompanied with a 3M sanding block and begin to level any areas that are raised and uneven. It will help if you have a bucket of water or water hose handy to wash away the excess material as you sand. Once you have completed the sanding with the 400 grit, you will need to now use a 600 grit sandpaper to go over the repair. The reason this is being done is to eliminate any scratches the 400 grit may have put in the repair and smooth it even more. You will continue this process moving up to a 800 grit and then a 1200 grit. Occasionally you will want to dry the repair area just to get an idea of the work you have done so far. At this point you can wax and buff the repair.