haha, resin outside and bondo over that is all you need to do. if you wanna smooth it out, you'll need more bondo for the lesser detailed pep files. i used the high detailed one (900+ folds, sorry, forgot by whom) since i had a weekend to spare. its best to experiment and see what best works for you. from my experience with bondo on cars, it might be all you need, if you can find a way to support your helmet/armor/weapon and not have it collapse on itself. i'm not sure it will adhere well to the paper/cardstock/cardboard well, however.
also, i'd like to add that, after a certain point, bondo can be easily carved after a certain amount of drying. you'll know its ready when you can leave a nail print in it and not have any of it come off on you when you remove your finger. then you can carve and mold away, then let it cure.