I used the Krylon Fusion on plastic motorcycle body-work. I didn't use primer, because that would've prevented the paint from "fusing" to the plastic. The point of this stuff is to melt a thin layer of plastic and fuse to it, so avoid the primer.
Just wipe down the plastic with some mild paint thinner, then spray.
As with all paints, several light coats are better than 1 heavy coat.
It wears great too. Even after dropping the bike in a sand gravel trap, the scrathes didn't go all the way through the paint. Then again, I don't expect your parts to go sliding through a gravel trap at 70 MPH.
Hint, don't use a halogen light to bake the paint. It caused it to dull while drying. Just let it dry in the air.