What car is this on out of curiosity? I think they're all plastic... what type of paint are you using? I have had good luck with Krylon Fusion on plastic parts - it sticks amazingly well and looks great. If you're using that, then I would just do whatever it says on the can for prep (not sure what it said - basically just clean the surface IIRC).
I just hope you didn't forget to Bulldog them though. I think that is what is helping mine stay together. I still don't have a single chip in mine. I am just waiting to see what happens the first time I peel out in gravel