I use Hot Wire Foam Factory cutting and scribing tools for cutting and detailing. Below is a corner tunnel I made for a client in California. The tunnel portals are sprayed with Krylon "Make It Stone" black granite. The flecks in the paint give it a texture and, since the propellant in this paint is not petroleum based, it will not react with the foam and damage it.
The entire mountain and rock formations are also made of pink foam.