Mr, Surfacer 1200 as a primer.

No, not the rattle can, but the bottle of Mr. Surfacer 1200.

Dilute 1:1 (or 50/50) with lacquer thinner and spray (airbrush) on.

After it dries, use the finest wet sandpaper you can get and sand smooth.

Since it's a lacquer based product, it will "bite" into the plastic and will not chip off.

It's also a great filler if you have some sanding scratched that you forgot to polish off.