Have you tried Newglass2? If you just have old dull gelcoat this stuff really works. I scrub the boat well with comet cleanser(gets rid of stains and removes oxidation). Rinse, dry, and apply about 5 coats. Just wipe it on. I do this at the beginning of each year and the boat looks like new. This stuff will seal the gelcoat to help prevent further stains. It is about 40 dollars a bottle and I can redo the boat about 4 times with each bottle. If you have repairs to cover this will not work but if it is just old looking gelcoat give it a try.
http://www.newglass2.com/cgi-bin/cp-app.cgi