Dicor makes a roof coating specifically for
Fiberglass.
I used it on a 13'
Scamp several years ago that had areas on the roof that the gel coat was completely gone.
I cleaned the roof with the Dicor fiberglass cleaner and then wet sanded it.
Two coats and it looked great.
I lucked out and found it on Craigs list for $25.00.
The gentleman never opened it.
I think it's about $70.00 a gallon at Camping World.
John