The best way to do it is to grind out each crack with a die grinder & fill with small strips of glass, fair it out & re paint.
It's alot of work for the little cracks but it's the only way to do it.
I learned the hard way that if you do it half ass, they will defenately come back.
A good marine fiberglass repairman should know the proper way to do it but make sure. Sanding/filling/painting doesn't work!