Right now I'm currently building a ODST helmet, using paper mache as a substitute base over fiberglass/resin. I'm using liquid starch(the stuff in the jug, not the can) as the 'glue', so to speak. Coat a strip of paper(1-2 inches wide, 5 inches long), remove any excess liquid, then place it so the paper is going from one ear side to the other(best place to start first is along the top of the helmet); Wait for it to dry(but not completely), then repeat the process again.
Do this across the whole helmet until it has at least 5 layers of paper, you are satisfied or until no light is visible from the inside. More than 5 layers is good, but try not to overdo it.
Only small problem I still have while doing this is the cardstock will start to sag a bit once you begin applying paper strips in the beginning; have something inside your helmet to hold it while you do this to prevent it from sagging.
(No pics yet, but will have some up soon)
I will say that if you are simply planning on doing paper mache and nothing else, it will not look as good as you might think. As far as the situation goes with putting bondo onto it, I do plan on using bondo but have not gotten to that point and will tackle that issue when it arises.