This is made from blue Styrofoam. No castings … just a little Spackle for the joints between layers, tempera powder, AI wash, latex paint and ‘floor sweep’. Posted a HIDI (How I Did It) if interested.
You can do it either way. The ‘dirt’ is zip texturing (I linked to an external article on zip texturing). Since that’s just tempera powder mixed with plaster it will ‘set’ on a wet surface. Me … I let the base color dry, did the other coloring, washes etc. Then, used a brush to wet just the horizontal surfaces. For a larger area you simply ‘spritz’ on with a spray of water. For that larger area the zip texturing in a tea strainer and tapping on it will sift the tempera/plaster on the surface. It simply hits the wet surface and what you get is a layer of colored plaster. The cool thing about that is that it creates a granulated look, much like dirt. For smaller areas like I did on the example where I just wanted it on the horizontal surfaces … like I said … wet it then dipped a a dry brush into the texture … held it over the wet area and tapped the handle with my finger. You get good control that way. You can go back at any time using that method even after it’s been finished if you want to add more.