1. To get good adhesion, you need a good sanding. Start off with about 250 grit and lightly sand, but get all the surface nicely scuffed. Lightly sand because you don't want deep sanding grooves. Finish off the sanding with 400 grit to get it a little smoother.
2. At this point, you have to decide if you want a good base or just move on to the graphics. You can apply a primer sealer, then white base coat, and then a medium clear coat. You have to resand the clear coat with 400 grit for your graphics work. I've painted graphics directly on the sanded surface before and if you sand it well, the adhesion will be good.
3. If you skip the primer sealer, basecoat, clearcoat step above, you can paint directly on the sanded 400 grit mask. Benefit of skipping step "2" is the helmet is lighter. Downside is, the primer sealer acts like a glue for the basecoat clearcoat. It's your call on this one.
4. I suggest Createx paints for the graphics. I used to paint motorcycle helmets and Createx paints are very good for graphics. I don't suggest the auto air paints and they lay down thicker than the standard acrylic line of paints.
5. After your graphics, wipe down the helmet with automotive cleaning solvent. This takes off any grease from the helmet.
6. Spray on automotive grade adhesion promotor. It's a clear spray for the helmet. After that flashes, spray on a catalyzed automotive grade clear coat. It must be catalyzed. Any uncatalyzed clear coat will not be as durable as catalyzed clear coat. I've used Sherwin Williams, Dupont, and House of Kolor clears. When I was painting helmets, the Dupont clear laid down really smooth. House of Kolor clears have a yellow tint in it, but I never noticed any coloring after applying onto graphics. Sherwin Williams is a little cheaper line. Two or three medium coats should be enough for protection. You can apply one extra one if you plan on sanding and polishing later.
Hope that helps.
