great advice Jack!

there are several products on the market that claim to be able to restore old gelcoat. some people swear by them. You can also wet sand the gelcoat (if not shot) to remove crud and restore its shine.

also there are many acid products that are great to clean the surface. many people in my area use sno-bowl (toilet cleaner). it is a good idea to wax the boat after using an acid wash to seal the newly clean boat. this will help to keep it clean.