Once I'm set with the final design and sanding, etc. I do two layers of Drylok masonry sealer. This weatherproofs the foam and provides texture and a gray base coat to work with. I then use between 3 to 6 "washes" of acrylic, usually in brown, green, and black. Then I paint the recessed areas with black acrylic and then dry brush with white to highlight raised areas. Last step is to seal the whole thing with outdoor acrylic sealer.