I personally use "Great Stuff" brand expanding foam for all my armor pieces. It works very well, is often very cheap, and dries very hard. The only thing is, the foam is highly unpredictable. I've been using it for at least 8 months, and I still haven't mastered the method. If you want to try it, I would experiment a bunch with it, before trying it on any pieces at all. And, if you need any help, you can pm me.
It's really not any more expensive than fiberglass/bondo. A gallon kit of smooth-cast 300 is about 90-100USD after shipping. That gives you 2 gallons of resin (1gal partA, 1gal partB)
2 gallons of standard resin is gonna run you about 70, then the fiberglass mat/cloth/whatever to use all that resin is gonna run you another 20-40 at least and all of a sudden you're about the same price as the smooth-cast.
Not to mention doing the smooth-cast is much much easier.
A trick i use with it is to use plain old elmer's glue as a release agent. after you get your smooth-cast to the thickness you want, just start getting the cardstock wet, it reactivates the elmer's glue, and you can just peel/wash everything off. leaving almost no residue on the plastic. After that's done, you can fill over any imperfections with more smoothcast, that way you don't have to buy bondo either. Just mix some baby powder in with the smoothcast (it takes around 1:1 baby powder to resin to get a good thick paste).