I know several like to use Testors Dull Coat (its a dull clear coat). I currently have not added any to my helmet or Jetpack. Still aways away from having a complete suit but will add it when I get ready to use the suit.
I used enamels and Testors Model Master Metalizer Sealer (Non- Buffing) over all of my paint. If interested check the first page of my costume thread for a little more about it.
It works very well...in my opinion.