you should palstic prime them and then if you want the smooth look you willl also need to filler prime them to smooth them out or they will look rough and imo poo
Sand the surface smooth first, and clean with alcohol to remove any residues.
Use a primer, then sand again with ultra fine 2000 grit (wet preferably), clean again with alcohol, then colour coat (at least three coats), then top lacquer (at least two coats).
The key to any good bit of paintwork is the preperation
and if your doing it in a garage or shed or whatever i would spray water on the floor and walls etc to try keep the dust under control, The finish you get will never look as good as if it was professionaly sprayed
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