If the bumper is relatively rough, and has plenty of repairs, I would strip of any existing paint, finish the fiberglass prep with 120 grt paper, then give the whole thing 3 coats of "Polyester spray filler" this will easily fill any pinholes, and any other sins for that matter, once dry, guide coat the polyester, and dry sand only 320 grt paper (Polyester must not get wet if possible) You now should have a perfect base for 3 to 4 coats of 2k Acrylic high build primer, Lesonal "Green" 2k primer is ideal for what you would want, and is easily sanded with 800 wet/dry ready for paint...
If you're confident about your prep work, just skip the Polyester process, and replace it with a couple of coats of Etch/Wash primer, this will help promote adhesion for the primer to the fiberglass
Thom
