Any good lacquer compound will work on old gelcoat. Sometimes (not always) the hull has to be sanded before compounding. For really bad surfaces I sand with 600 wet first...some people use 1200 and work longer. Then compound to get the scratches out (DuPont 202 is what I used in for fiberglass production work). Then use a non abrasive polish to touch it off. I normally use a commercial duty 2 speed 8" grinder with foam pad for sanding, compounding and polishing. The foam pad makes the job easier and more foolproof. I recently bought a cheap variable speed polisher/grinder on sale at Harbor Freight for $25. Variable speeds make gelcoat work more user friendly compared to my on/off tool. Attachments are limited but it's an excellent tool for the buck and great for hull buffing. Bill P.