Is it allowed to airbrush a helmet?

Most brands allow their helmets to be airbrushed as long as one doesn't penetrate the top coat when sanding. Arai for instance mentions it on their website (arai site ). Dutchairbrush does not paint Rubatone helmets (helmets with a soft surface).
When in doubt contact the manufacturer of your helmet

What is possible?

The work can be as wild and intricate, or as simple and clean, as you desire. The customer’s budget and timescale are basically the limit. Your airbrush can be anything you can imagine. You can send us your own idea, photo, drawing, cartoon or an image you want us to duplicate (bear in mind the result will always be the artists interpretation of the image).

Prices

It is hard to be specific about the price of your dream project without some knowledge about what it entails so an estimate can only be given after you went over the project with us and we have a clear idea of the time entailed in your design.

An indication of the prices for a helmet or motorcycle part (ex tax) is:
-Base Color helmet: €100.00
-Starting Basic Graphics (inc base color): €225.00
-Sand and Prep used, painted, or production helmets to paint new design: €75.00
-Price per name on helmet: €50.00
-Price per set of numbers: €40.00
-Clear Coat, Polish: €150.00