is it really Styrofoam that you are working with? The white bead like stuff? I suppose it doesn't matter. EPS or Styrofoam.
My observation is that style and desired result require a small arsenal of tools. Various blades, knives, rasps and a hot knife and a hot wire cutter all come into play.
harbor Freight has the Hot Knife as one of the weapons.
I have used kitchen knives, a machete, electric carving knives, a knife edge blade for a jigsaw (Bosch makes it). I have cut/carved mainly EPS and Great Stuff. I have seen any and all used. Some appear to be a better application for creating rock like surfaces, while others are great for general cutting, shaping. Heck, a hack saw blade with tape on one end for handle is dandy in some cases. A pencil is great for stone outlines and mortar joints.
Cruise the scenery forum and check out how folks get the foam to look like what they want it to look like. Hit your tool boxes and grab anything and experiment.
