Hi! I too use styofoam and plaster-of-paris. The POP sands well and takes paint really good.
I have heard of "Mountains of Foam" but never tried it. I have heard that you have to be careful with how much you use as this stuff will expand, and expand, and expand, and...LOL!
Someone tried to use it as a filler for a vinyl figure to make it more rigid and stable. Split the models legs open.
I think it dries as hard as a rock too. Not easy to shape after it sets. It's probably like that spray stuff they use for insulation in walls, etc...
James