If you can live with a slightly smoother surface, then there's another way to go with it that's a little less labor-intensive and about a zillion times more durable: once you've got the sculpted piece ready to roll, you can take it to a place that does spray-on bed-liners for trucks and have them put a coat or two on it. I've used this for some carved-foam pieces for work purposes, and it works great: extraordinarily durable (the final product is nearly unbreakable) and holds paint pretty well. It'll retain the nooks and crannies of the piece, but it will dull the edges a bit.

check out how joemichaels did his arctic piece using styrofoam. I wouldn't sculpt over it at all, I'd build stacks of foam up (more like the insulation type of foam), then carve down from that, rather than use something else to build up from it.