Can I Paint My Bicycle Helmet?
Summary: Painting a helmet may damage the shell or the foam unless you are sure that the paint and helmet materials are compatible.
Paint varies widely. Some paints have solvents in them that improve their grip on the surface and aid the drying process. If the solvents are incompatible with the helmet shell material they can possibly attack the shell, weakening it. Plastic can "craze" with jillions of tiny cracks running in all directions. The damage may not be highly visible, particularly since it is covered by the paint. And the weaker plastic may or may not ever make a difference in a crash, depending on where you hit.
Paints and Plastics
Not all paints attack helmet shells. Some plastic shells are actually clear, and are painted underneath with paint that is compatible with the shell material.
To make sure there is no problem, you need paint that is compatible with the shell of your helmet. If you are dealing with a single brand and model of helmet the manufacturer should be able to tell you if the particular paint you want to use is compatible with their shell.
Spraying paints complicates the question, since "overspray" is likely to contact the helmet's foam as well as the shell. Almost all foams used in bicycle helmets are damaged by petroleum-based solvents found in many paints. (You can prove this to yourself by putting a chunk of foam in a small amount of gasoline. The resultant goo makes are really strong adhesive.) Even if the paint is compatible with the shell material, extraordinary measures would be required to keep any overspray from contacting the foam at any point. So our recommendation would be to find a paint that is compatible with both the helmet shell and the helmet foam. Unfortunately, we are not qualified to tell you what paint that might be, and can only repeat the advice to contact the helmet manufacturer.
Stickers are a closely related subject, and we have a page up on using helmet stickers. We have another page on our experiment to find out what common cosmetics and insect sprays attack helmet materials.