Thanks for the props blueboy.. I try to do what I can...
anyway here is what I do, step by step. May not be the professional way, but it works and looks good.
Sand down plastics with 150 grit sand paper, they will looks kinda hairy, thats good.
Spray with adhestion promoter, and then put down first coat of primer...
Wet sand with 600 grit paper, may sand through and still look hairy but that OK.
Prime again, and wet sand with 800. May still look a little hairy, still OK.
Prime again, and do your final wet sand with 1000 grit, should be smooth, and no hairy feeling left. Look like this...
Now lay down your paint. I use Krylon Fusion becasue it is flexable and doesnt crack when you bent the plastics, good stuff. Here is mine after the paint laid down...
Notice you cant see the crack in my front piece anymore, bottom of post tells you how to fix that.
Now if you plastics are really scratched up, or cracked you can fix that too. My front was broke and I just epoxied the crack, and used a plastic body filler on all the deep scratches and over where I epoxied the cracks.
Here it is with the crack fixed, right between the louvers...
Here is the finished product...