Painting polystyrene? Styrofoam?

I have a huge polystyrene ball of 24" diameter. In case you didn't know, polystyrene is a sort of styrofoam, but not the rough, scratchy kind, it's the smooth kind that is sometimes used in packaging. Anyways, the ball will eventually be a mask/head for a mascot-ish costume and it will need to be painted at some point. Although I will try not to hit it against things, it will definitely be handled a bit. I'll be wearing it, carrying it, setting it down on things, putting it in the back seat of my car... That said, what kind of paint should I use that won't rub off, chip off, scratch off, etc just from regular handling? Is there a paint with some sort of elasticity to it? Not talking about latex spray paints, I'm talking about the kind of paint I'd brush on.