1. Use isophathalic gelcoat. Do not use orthophathalic gelcoat. They do not hold up as well. Fortunately they are not common, but I have seen some low priced gelcoats that turned out to be orthophathalic so buy a premium well-known brand.
2. Skip the peel ply. Do like the pros. On the last coat of gelcoat before you start sanding, add wax to the gelcoat before adding catalyst. The wax is sold under different names (Modifier-C, Sanding Aid). The wax will inhibit the oxygen in the air from reaching the surface of the gelcoat and allow it to thouroughly harden thus not gumming up the sandpaper. If you must recoat, sand thouroughly and clean thouroughly before reapplying more gelcoat.
3. Get a catalog from Fibrglass Coatings Inc. in St. Petersburg, Fl (sorry I don't have the address at the moment but they do have a website ~
Fiberglass Coatings Inc. Your Single Source for All Your Fiberglass Needs ). They sell everything that the profesionals use and in the catalog is an instruction manual on how to do exactly what you are attempting. It is the best quick and dirty guide to working with fiberglass I have come across. If you follow their guide your repair will come out looking as good as a professional.
Richard