Thanks T, I had forgotten about the idea of using foam for rocks, because that one was the only one I've done, and all I did was paint it. I have a 20' long cliff to make and had planned on using plaster, figuring I wouldn't be able to get foam to look right, I thought I had just been lucky. Reading your procedure has made me decide to try foam first. And makes me feel more confident it will work out well.
Gary