hey Sho!
Depends on the part... I find that a lot of citadel paints will slide right off some plastics. no matter how much I wash it- something about the plastic just doesn't jibe with the paint. In those cases I'll hit it with some folk arts flat black (that stuff stick to anything) But typically- unless I'm painting white/yellow/ or something really bright- which I'll give a white base coat to- I don't primer much at all.