Painting a Helmet

[that-redhead.livejournal.com]

I'm looking to paint my roller derby helmet, but I have no idea what kind of paint to use. I want it air brushed, but I just can't afford it. If I used acrylic paints, is there a varnish or something I can put over it to protect it?

If it helps, my helmet is a glorified plastic extreme sports helmet.

Thanks!

xo,
C.N. Red