I'm actually not clear on the question:
Are you looking to paint
the insulation on the inside or the fiberglass on the outside???
As elsewhere - I used spray vinyl (comes in cans from Rustoleum for repairing car dashs and such). Worked very nicely.
I used Rustoleum paint and rolled it on. I'm not totally pleased with the texture that was left to be honest though. If I were to do it again I think I'd definatly use a spray (for something flatter), and possibly switch to either a polyurethane paint or gel coat itself. Both of those options would be MUCH more expensive however and require extensive prep work as well. The nice thick, mildly textured Rustoleum did a nice job of covering up 31 years of weathering flaws and some fiberglass repairs that I also did.