I very recently sculpted a 30 foot tall Gatorade bottle for a client and before I molded it, I hardcoated it with Styrospray. It is a good product. It brushes on good but it also sprays really good too. I used a hopper gun and a gelcoat cup gun with good results. It is self leveling and requires no sanding between coats and can be recoated almost immediately. It is a moisture cure so you need to be careful with the spray equipment and how you clean it.
On foam, it is not completely ridged. It is hard but it does have a slight flex to it. It is a ***** to sand! Also, you need to prime it BEFORE you use any polyester filler putty or it will flake off during sanding.
If you mix up too much, you can put it in a cup or bucket, spray a blanket of nitrogen gas over it and it will still be liquid in the morning!