I airbrush.
Once you get into them they get very specific, especially paints. I have no idea how much art background you have, but that can be a pretty big factor. They are odd at first, and once using one for a bit youll have a crazy appreciation for some of the stuff you see on helmets. I have a fine tip badger, cost me about 200 bucsks. I got this one cause its pretty generic and works well on canvas which is what i do. If you wanna get into painting helmets, i would definently suggest getting a very detailed brush. Also, like anything, PRACTICE. paint is pretty cheap, so just go to town. Im not really sure what type of base youll want to use for painting helmets, but for all my work i use a water base, then a coating when im done.
Get a real air compressor, not the little dinky ones they sell at the art store, being a sledder im gonna assume you have one but who knows. You can find a small good one at home depot for not alot. Also, just make sure yuo were a mask, ive had many airbrush days when i didnt wear one and when i blew my nose that night some intersting colors came out.....ew.
All in all, airbrushes are a blast, especially once you begin to get the hang of them!
feel free to PM me about it anytime.....