You have to use a paint
that is specifically designed to go onto fiberglass, like a marine paint for fiberglass boats
I don't know that there are any Pittsburg or duPont products that would work well but, that said, Interlux Briteside is pricey, about 2-3 times the cost of Rust-Oleum marine paint, but worth every penny. You can look up prices on-line or contact a local boat supply shop.
BTW: You will see a lot of posts telling about "some" rust-oleum jobs that were inexpensive to do and came out well. The operative word is "Some".
I have never seen a comment on the Interlux product that was less than "beautiful"when the proper prep, undercoat and top coats were correctly applied using the roll & tip method recommended by Interlux.
I think I spent about $300 on prep, undercoat (2) and finish coats (3).
FWIW: Auto painting
shops typically quote about $1200 to do prep & spray a Hunter, depending on condition.