What I did/am doing with my helmet is :
2 layers of resin on the outside
1 layer on the inside
Fiberglass the inside with an additional layer of resin on the inside
Do a basic sanding all around
Bondo small parts at a time, dabbing on more bondo than needed to 'chisel' away for details
Sand with fine sandpaper
Metal glaze parts that need to be smoothed out further
Paint
Add LEDs, visor, cooling system, etc.
And that's pretty much what I plan to do. It also depends what helmet you're doing, if it's Flying Squirrel's or Slyfo's, you might not need to add a lot of bondo for anything other than smoothing things out. You might even be able to go straight to metal glaze or fiberglass jelly (if you're doing that sort of thing.)
Anyway, hope this helps!