Painting should be undertaken by an experienced painter. It is quite difficult and probably not that easy to do at home without experience and the needed materials. The process is similar to airbrushing plastic parts of cars (bumpers/fenders etc):
1. The surface must be roughened.
2. The whole surface must be degreased.
3. Apply a full surface special feeder.
4. The helmet can then be painted with a two-part paint.
5. When the paint is dry the helmet should be sprayed with transparent varnish.