Beg to Differ! Order paint, clear coat (I ordered an extra can) and plastic adhesion polymer form
http://automotivetouchup.com/ They mix it just before they send it. But if you want any local paint shop will mix it and put it in a spray can for you.
Get some 400 grit sandpaper, a bottle of astone (sp). Lightly sand part, Tape off whatever you don't want painted, I removed most of the parts I painted. dampen a lint free cloth, I used paper towels, with Astone, wipe everything down, Do Not Touch it with your bare hands. I wore rubber gloves. Read the directions on the can, lightly spray the plastic with the polymer. (by the way, auto parts stores sell it too, about the same price). I put a couple of coats on. Don't rush, i did on one piece and ended up repainting.
Layer on 3 or 4 light coats of base coat, car color. Let this dry for recommended time between coats. Let it dry then start with your clear coats. Inside the car I did 6 to 8 coats letting it set about 6 hours in between. In the engine compartment, I put 4 coats.
Remember, if the piece has a rough finished it will be rough when your done. DO NOT try to smooth it out with paint, will take a lot and be wasted because it'll never dry properly.
Or you can skip all this and take it to a paint shop.