You need to buy my book (when it comes out).

There's two whole chapters on repairs.
Basically if the gelcoat has cracked it means that the grp underneath has flexed or cracked as well. The first thing you have to do is to reinforce the underneath with two or three layers of csm so that it won't flex again, then you have to scrape or grind out the damaged gelcoat, repair any damage to the newly exposed fibreglass and paint in new gelcoat leaving it just proud of the surface. If you tape a piece of polythene or polyester release film over it to exclude air then it will set solid rather than sticky. Finally you have to sand and polish the repair so that it matches the original surface.
This is the crack opened out and new glassfibre laminated in to the area. It will be ground down below the level of the surface.

This is the new gelcoat being painted in to the repair.

This is the completed repair.
