preg the plastics really really good. Got to your local auto parts store and get a good cleaning colvent for body work, like prepsol or duplicolor prep solvent or laquer thinner for thinning paint or cleaning a spray gun.
Get your self some sand paper or scotch bright sanding pads like 220 grit and go over the plastics with that to scuff them up ( dont sand the hell out of it) just go over it enough to take the shine out of the plastics)
then clean the plastics with the solvent, dont be sfraid to slop it on there you want to get as much of the wax/ or film off the plastics. Let dry...
now for paint i personally would use a good primer for the plastics like the krylone fusion primer ( dont like the end result of the krlone base coats though) get your self a can or two of duplicolor engine enamal, do 2 really realy light coats to get a light layer of paint on it and wait for it to tack up and start to dry, than paint it like you would spray anything else.
If you take your time and prep good and take your time and spray them in a good manor ( sid eto side sweeps) you can get spray paint to have areally nice finish. and with the engine enamal you dont need a clear since its made for motors and will hold agood looking finsish for along time
hope this helps ya save some cash and good results