I remember painting some massive crown mouldings made of fiberglass a while back. It was spray primed,if I remember correctly, and we used Nova Color acrylics which worked really well. Sorry I don’t have more detail beyond that.
When I painted fiberglass for the cruise ships, we lightly sanded the fiberglass to give it a tooth for the paint to stick. I don’t remember if we used anything special as a primer after that. Sorry for not being extremely helpful.
I have painted on lots of fiberglass. First wipe it down with acetone ( will help remove mold realease) will soften to accept primer. Sand if you can for a tooth. There is a product call Bondz made by zinzer (waterbase, for hard to adhere surfaces) after it drys you can use nova or whatever you like. Let dry and seal with a good clearcoat.