....One thing that is common with either Styrene plastic or resin is that both are petroleum based which means that with the porous surface of a body, the paint is trying to seep into the body as oils are trying to escape (come to the surface)....once a styrene plastic body is totally void of any and all petroleum based products, it starts to become brittle. Resin, by nature, will have a higher percent of oils, hence the first thing you do before anything else is soak it to remove enough petroleum byproducts to make it paintable.
So lets say for the sake of discussion that you have laid down a good coat of primer....the primer of course gives the base color coat something to stick to ....and you shoot it with color that never seems to dry right. Not all paint is compatible with every other paint....do your research on what works best with what. I've had good luck lately with Dupli-Color sandable automotive primer....I can either shoot it straight out of the rattle can or decant it for the air brush....now this is not to say that the next builds color paint choice will be compatible with it or not....your mileage may vary.
What I have started doing to see if everyone will play well together is to use a plastic spoon (same as #6 Styrene) and spray the paints...primer and color...to see if they dry correctly, no bleed thru, etc etc.
I have 4 other different primers that I could use to get the right combination...Testors, Krylon,Rust-Oleum Automotive Primer and Rust-Oleum Filler Primer.
I have had in the past tested the paints with decent results, but when applied to the body the result was bad.....that one I have yet to determine what happened.