how to finish polypropylene?

Submitted to Product Chat

I'm working on a new robot, and I'm wanting to keep the original body/innards cover, because it actually has a pretty cool shape. It's made of polypropylene. I've already sanded up to 400 grit wet to get the scratches and gouges out of it. What will stick for a primer/basecoat? I'm wanting to use an airbrush for the finishing coats so that's something to think about too I guess. Thanks in advance!