Yep. Just the nooks and crannies for a build like you have.
Those people that you see putting bondo all over their scratch build, are generally making their helmet out of some sort of heavy card stock, or chip board, then coating it with fiberglass resin. After that, you begin the tedious process of covering it with thin layers of bondo, then sanding it down. Rinse and repeat as many times as it takes to get it smooth.