/cgl/ - Cosplay & EGL

I will benefit from the new bigger venue I think.

I don't know about anything besides cardboard, seeing as I've only ever used cardboard (and fiberglass), so can't help you there.
One general rule I found for cardboard is that you never want to paint it directly. The resulting paintjob will look ass-ugly. You wanna cover it up with poster stock to give it a nice smooth surface.
Also, NEVER make your costumes with duct tape. One, it's bond gradually weakens. Two, it is hard to paint or glue anything over it (extremely non-sticky surface).
Use hot glue instead. Less conspicuous, stronger, and paintable. Just down burn yourself like I did multiple times.

Fiberglass is good for making free-form parts. You just gotta make the shape over which to apply the resin. I make a ribbed frame, into which I sprayed expandable foam. After giving it AT LEAST 3 days to cure, I cut and shaped it to the desired shape, slapped paper mache on it and finally applied fiberglass resin (be sure to use fiberglass mats, coat it multiple times and sand it down really good).

Pic is a mecha cosplay I'm currently working on. Built entirely out of cardboard and some fiberglass parts.