What urethane resins have you used? Smooth-On's Shell Shock, for example, can be applied directly to foam, with no barrier, and no distortions at all.
Regarding fiberglass resin (more accurately, polyester resin), you might be mixing a little too much catalyst. The more catalyst you mix, the faster it will kick off, and the hotter it will get.
Personally, I've moved to using PU and epoxy resins, so I haven't tried polyester resin over foam, but from what i hear, a couple coats of gesso should be enough to prevent heat distortions on most all but the really pointy bits.
Usually, if it does distort, it just makes some air pockets. For props, this is often not a problem, it just makes thing a bit less sturdy. If this is important, you can drill a pinhole into the distorted area, and inject a touch of gorilla glue. It foams a little as it cures, so this should fill the void.