There's multiple ways you can achieve this, but the best way is gonna be dirt itself. Obviously you realize this is a working model, so dirt will naturally build up on it depending on where you drive it. But if you really want a shelf queen that has permanent mud, then you can purchase specific modeling resin to bind with dirt, sand, micro pebbles, etc.... in addition to dry pigment or paint.<div></div><div>Theconvenientmethod is getting resin medium at the local arts and craft store. You can use either gloss gel or matte gel which dries nearly crystal clear. Add dirt and coloring to this to taste and it will take an upwards of 1-4 hours to cure. Matte would be best for a dry mud and gloss would give a more wet feature. Depending on how liberal your mix is, this will alsodeterminehow wet or dry looking it will be.</div>