I think if you sand it, the primer doesn't matter as you'll give some "tooth" to the primer for your basecoats to hold onto. If you aren't sanding, you want to use the primer your paint system recommends so you can get a chemical bond. Probably more a concern if you use a primer sealer, since you'll most likely sand you primer anyway to get it nice and smooth.
An epoxy primer should give the best adhesion :biggthumpup: but if you sand your fiberglass thoroughly you should get good adhesion (don't leave any shiny fiberglass!!!:twak
