Most motorcycle helmets come already painted---that's why you don't see bare metal. You don't need to scour all the paint off before you can add your own paint job, but you do need to get rid of the clear varnish finish on the helmet before you can go to work on it.
Things You'll Need
400-grit and 600-grit sandpaper
Newspaper, garbage bags or other plastic sheeting
Flexible stencils (optional)
Remove all the screws, padding and accessories from the helmet that you can. Anything that won't come off must be taped over with painter's tape. You should also cover the eye opening with painter's tape to keep paint from getting on the inside of the helmet.
Wet a sheet of 400-grit sandpaper, and scour the helmet until most of the clear varnish coating is removed.
Continue sanding with 600-grit sandpaper until there's no trace of the clear varnish.
Spread out newspaper, garbage bags or other plastic sheeting to absorb any paint mess.
Apply a base layer of airbrush paint to the helmet, and let dry for a few minutes.
Airbrush any designs you may have worked out onto the helmet, working one color at a time. Flexible stencils---shapes cut out of flexible plastic or paper---may be helpful to guide your airbrushing. Consider taping them in place with painter's tape as you work.
Allow your airbrushed designs to dry, then follow up with one or two coats of clear-coat varnish to seal and protect the designs.
Remove the tape from the helmet and reinstall anything you removed at the beginning of the process.
Make sure you're working in a well-ventilated area and wearing eye protection before you begin work. You might also consider using a dust mask to keep paint dust from getting into your respiratory system.