I'm just starting the paint process and know next to nothing, but I would probably epoxy primer first and then the fiberglass...
I hope others chime in on my comment to either confirm or deny because I really don't know for a fact and I'm talking about but I keep thinking of sealing the metal so if water got in between it and the fiberglass it wouldn't start rusting...
Yea your right. I sprayed everything down with Rusteoleum primer after I cut all the rust out. Im sure a good coat of epoxy would be best. And I'm 100 percent sure that fiberglass is the wrong way of doing it lol.