Hi: Steve in UK... At all costs try to revive the finish of the GelCoat. No matter what product you use, to do this, you will also knead(sic) gallons of "Elbow Grease"... and plenty of time to apply it. A large bottle of linament will help to finish off the "Elbows"!!!
BTW... WELCOME to the wonderful world of fiberglass.
Alf S. North shore of Lake Erie