spray paint is made to be easy to use and still look good, they acheive this by putting acids in the paint to eat away dirt and impurities in the thing being painted, but when you put in on helmet plastic it will eat away the soft top coat of the plastic and maybe the whole shell. definately don't use primer(has more acid than regular paint) and buy paint made for plastic from your local auto store, its more expensive but it won't compromise your helmet and it will last longer without scratching