I need to look it up again, but when I did stuff in my sculpting class, there was a certain type of foam we used and something we used on top of it and it essentially turned it into something that was hard as a rock and could be painted. It was extremely fascinating, but it's been a year and a half since I last took the class so I can't remember for the life of me what was used. I'm sure I could easily google it up. I was able to find everything I needed at a hardware store for it though.
Hmm thing's I've made have been a life sized wolf, a 1 foot tall full spyro sculpture, a "wukong" sculpture that was about 5 inches tall, various polymer Pokemon sculptures. I've also experimented with cardboard and I don't remember the name of the stuff we put on it, but you could essentially mold it and it would turn out smooth and as hard as a rock as well. I miss that class. Man I hope I can make something again sometime.