A suggestion, Mark....I always resisted the WS plaster cloth on my O scale layouts. I used wood, window screen, joint compound, and staples. With my new N scale layout, the extruded foam went perfectly with the plaster cloth, making it easy to shape features. Like you, I got rid of most of the white styrofoam. Too messy, difficult to carve.
I put all kinds of light weight stuff under the cloth to form the mountains. Put it on wet, push-pins to shape it, brown paint for ground cover. Used my old rock molds with sculptamold, carved many of the faces with a simple knife. Then, just added multiple textures and colors, like all the guys do.
Anyway, just a thought. Wanted to let you know that, with this stuff, it was easier than it used to be!
Best wishes,
Jerry