If you use RNB (which is kind of tricky to use), then you should practice on some scrap pieces painted exactly how your helmet was. If you've already used RNB a lot then you may not have to practice. Putting on two coats is best because the first time you may end up with a lot of uneven spots. The second time you can even it out as needed.
As for a clear coat, I believe it is unnecesary.
Heres a link on RNB where they mention clear coating.
http://tdh.prop-planet.com/viewtopic.php?topic=3454&forum=9
Good luck, i hope this helps