I already posted this over at the RPF, but thought I'd post it here too: How to plastic coat foam costume pieces
I've been doing some testing with plastic coating plastazote foam, although I've only tried it a couple of times, I'm pretty sure it could give some amazing results with practice. Essentially this gives you the speed and ease of foam building with the finish of plastic or fiberglass pieces.
Here is my foam test piece:
It also works for expanded polystyrene and other materials, so I think my next project will be a Terminator Endoskeleton made out of foam, carved polystyrene, cardboard tubes etc, all plastic coated and painted up like metal.
Here's an article with words: XRobots.co.uk/coating
And a YouTube video showing the step by step process: XRobots - How To plastic coat foam & expanded polystyrene prop and costume pieces - YouTube