You need to make sure the shell is washed completely and then use the correct lexan paint such as Faskolor or Createx / Auto Air ( which is all Faskolor is anyway ), these are all airbrush paints, there are plenty available in Aerosol too, I've used Tamiya and always had success with them staying on, I can't comment on others though as I usually airbrush mine.
Jon at RCS sells all this and has tutorials on his site
http://www.rcsgraphicworx.com/
Or Terry just wrote an article in Radio Race Car ( May issue ).