Some folks DO use silicone. However I find it makes me feel really ill when used in such large amounts, and the viv stinks for weeks afterwards. This is why I use Gorilla glue. It cures VERY fast (100% in 24 hours), DOESN'T make me feel ill whilst using it, and doesn't have any smell that I can detect. It also has a consistancy like golden syrup, making it easy to apply, and foams up like foam allowing some interesting sculpting by pushing it back down etc. Also you don't need to put anything on it to attach substrate to, you put the substrate onto it directly once it's foamed a bit.
Some people have used Drylok to seal styrofoam, you can also use some acrylic paint to tint it to the preferred colour/shade and it makes it waterproof too. Check out a recent thread in the DIY section about a tree butress, Ryan used drylok and it looks good