The higher the def is, the less bondo you will have to use.
Use bondo on the areas you want smooth, but usually thats kind of the whole helmet (kind of!!!)
on some parts there are like teeny tiny cracks, valleys and holes you dont see unless primered, you just put on a little dollop of bondo to make it look cleaner.
My first resin helmet needs bondo on most of the surface (even though it is Flying Squirls model!!)
My resin was just so unclean, my pep wasnt very good and thats why I had to use a lot of bondo, more than healthy, hours of sanding.
Anyway, good luck with yours!