A lot has been made about this subject, tried&true,old schoolers swear by a
sealer coat of shellac, that works ok, but I have no problem with plain old Krylon char grey. If the part is bubbly I'll hose it(Detail permiting.) mush my thumb in it, coarse sand it feather on several more coats, presto bubbles gone, mostly!. Krylon builds up quick and dries fast, nuff said.