Just want to suggest this: Though I haven't as yet done it myself, if you're talking mountain rock formations and you've already given them the crevices they need to look like rock, use inks, not paint. Black and brown inks for the crevices and light gray for the surfaces. It's more like staining than painting....
Hydrolcal can be colored
with mineral spirit and oil paint washes. It is more of a staining
procedure that was mentioned earlier. water based washes may
I KNOW mineral spirit washes and oil-paint work flawlessly
burnt umber and gray oil paints mixed separately to make a wash
basically staining the light weight plaster.
This is called the hard shell "scenery technique"
introduced by Mr. Lynn Westcott back in the 70's around this time.
the "hard shell scenery" technique"
Rock castings are also employed here.
It can be a messy technique though. But very convincing.