I don't know if this will work, but I have thought of a method of shapeing the helmet by filling it with foam after the outside fiberglassing stage. I haven't tryed it yet but here is some things I know about the foam im useing:
-It is named "fugue foam".
-It almost triples it size when it is cureing (fill 33%), so it might tear your helmet apart
I might recommend you test it with a siple size tester helmet before you decide to try it out on your h3 pilot helmet
I don't want to be the reason your helmet got destroyed
Since I haven't tryed it yet, I dont know how hard it is. If you saw how they created the Halo Reach Noble Team Statues, they used hard foam that was cut out by a cnc machine. But for you, the pep will act like a mold for the helmet so you don't have to use a cnc machine
You just sand the trought the paper and start to add all the detail
So here is one of my methods im planing to use for my future prodjects
I haven't tryed it yet, so it might fail
But At least I think it is a little better than useing hours to sand super hard bondo...
Btw. If you live in the us, I might recommend you to go to your local hobby store and by some smooth-on foam. It is meant to be sculpted
I saw Benstreeper do it^_^