I personally use this:
http://www.duplicolor.com/products/bumper.html
I used it last week to paint the grip and stock of my AK (ran out of time to paint the rest) and it held up exceptionally well. I even took a direct hit to the stock and while it did leave a slight mark, there wasn't anything major like chipping or spider-webbing.
Definitely rough up the surface first though, I had used a old set of modelling sandpaper and they worked pretty well.