Question: Did you paint this helmet with rustoleum pro finish white? If you did you have to wait a week before you sand it and paint it again. If you try to paint the helmet before that paint completely cures, you will get orange peel. You can also get orange peel from the surface not being prepped and oil free. It has to be pretty well prepped. Lightly sand it real good, primer a few coats and then use a 1000 grit sand paper to finish the final wet sanding. Scan the helmet for areas that need more attention. I use a good tack cloth to pick up all the dust for the final wipe down. Paint in an area with no wind, no bugs and no chance of the helmet falling. It should turn out pretty nice. None of my buckets have been flawless. There is always a small "something" that bugs me.
Rustoleum Pro is a great paint but it is very finicky. The Krylon stuff is easy to apply but you wont get the gloss Rustoleum provides.
You can wet sand out orange peel with 1000 grit and alot of patience as long as the paint is thick enough..... If you burn through the paint though, its over and you start again anyway. Once the surface is smooth again, you can buff it out with novus or a decent buffing compound or car wax. The finish will be glossy but not like the Rustoleum finish. You may need to wet sand the entire helmet to get an even finish that matches...