Styrofoam can be tricky, as many types of paint will actually eat away at the material. But if you cover it with something, I think that certainly could work... paper-mache if you're looking for a simple project, fiberglass if you're a little more ambitious.
We made Integral's Tifa
armor using paper-mache on a small balloon so it was more rounded. Then we fiberglassed over it (just two thin layers will make it nearly indestructable), put body-filler on to smooth it down, and added the rivets. We just used standard silver-metallic spraypaint, and then Integral put a black wash over it... the black wash makes ALL the difference in the world, and the first thing I said when I saw it was "Man, I wish that Cloud cosplayers would use this!" ^_^
Good luck, hope that helps a bit!