Yep, resin and bondo will do wonders. That's what I used on my quick-and-dirty shoulder pieces and they turned out pretty good. Not only does it take paint better, but it also give a nicer texture than cardboard will, at least to my eyes.
It was just cardboard hot-glued together and bondo on top. Didn't even bother with straight fiberglass resin. I put a few strips of fiberglass on the back to give more support to ensure things wouldn't break, but you should be able to find other ways to reinforce whatever you're building if need be.