I modified it slightly by using vinyl spackle (its more flexible) and adding PVA to the mix. To paint them I experimented with paints while they were wet, and woodland scenics earth tones when they were dry. for a bit more durability, they were sealed with a water based matte varnish afterward.
Of course, I also had to experiment with other ways to change or improve this idea, because thats what I do :)
This is the above technique, just with heavy amounts of wall spackle in places. Works well, just not as durable.
This one was coated in cheap white paintable caulk. Two coats, Mcfly.
The result? A super flexible rock that can be literally thrown against the wall without damage.
The only downside is that caulk and the scenics pigments diddnt hit it off together; I had to use paint and drybrushing to get it even remotely looking like a rock.
thats all for now, Rock out with your... rock out.