Krylon now makes a spray pant just for plastics. It's called their Fusion line. I have not found anything in a tin that will stick to plastic in the long run. I have a custom painted squirt helmet from back east. It was air brushed, clear coated and baked. It's the same process they do for motorcycle helmets.
What type of paints/clearcoat are you planning on using? If you want to use acrylics-XIM makes a bonding primer in a spraycan that I use on plastic all the time, no helmets yet. I get mine at a paint store. It comes in clear and white. Scuff the surface very well and clearcoat it well and you should be good enough to use acrylics. Add some Golden Gac400 to your acrylics, it helps bonding on hard surfaces. Auto paints work great but that is a whole other chemistry project.