I like the Evercoat Featherfill G2 poly primer for fiberglass. Not expensive compared to a Spies, Upol, Standox, etc poly primer.
I would also recommend not cutting any of the gaps until you have primed and blocked it 1 time. You will be amazed at how "not straight" it is once you start sanding the primer. All the little micro lows will show up, and don't forget the pinholes...
