I always get my fiberglass work all done then spray a nice coat of gel coat over everything before spraying any type of primer, etc. This seals the fiberglass up perfectly. I have made several consoles from scratch forming them with only fiberglass, then spray my gel coat, then do my bondo work, then poly primer to make ready for paint. Also do this over any fiberglass repairs before adding any bondo or primer or paint. On complete paint jobs on old Corvettes I always spray all new gel coat on the complete car to get all the old and new fiberglass sealed perfectly so no stray hairs stand up later on the glass and poke through the paint as I have seen on many jobs that just primer over the fiberglass. I get white brushable gel coat from our auto body supply house which also does a lot of sales to boat repair businesses. I thin it with acetone and you can then spray it to whatever thickness you want.