smoothest stuff i have found is the cheap stuff from walmart, i use it for everything non automotive
If you are using sandable primer, remember that it is probably 40% talc, thats why is is bumpy, just build up the coats then sand the top of it off, its a waste of time and paper to sand in between coats on the primer
If you are doing a nice fiberglass helmet or FG armor i would recommened a sandable primer sanded smooth then use some autommotive epoxy primer as a sealer.
[qoute]For a finer spray place the can in a bowl of hot water for ten minutes before spraying. The heat causes the paint to come out much finer.
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the outside temp should be pretty hot as well for this method