I'm not sure if you lot are aware but the stuff i use on my Nitro racing cars is polycarbonate paint which is available from any "proper" radio control model shop (not argos etc etc), if you have a good shop then the choices are fantastic, candy's metalflakes, pearls, etc etc.
This stuff is specifically designed for spraying on the inside of model car shells, it actually bonds itself to the plastic but remains flexible and leaves the outer surface completely as it left the factory so you can still wipe paint after a hit but the hit won't crack as the shell flexes.
http://www.parmapse.com/faskolor.asp
The choice is endless, so lets see some trick hoppers people !