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.
The part where you sprinkle on the dirt after you pained the foam, do you do this after the paint is dry or while it is still wet?
Really nice technique and results. Thanks for sharing. I added the link to my references on foam rock carving. Thanks.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
Amazing work. Thank you for the link and the method.
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.
http://www.etraxx.com/phpbb3/viewtopic.php?f=23&t=31