For washes, I take several approaches... 1. I generally paint with one media, such as an oil-based paint, then wash with a water-based (acrylic) paint. This will contribute to the "pooling" of the wash in low areas. 2. A gloss enamel and an acrylic wash will concentrate the pooling further; a flat enamel and an acrylic wash will spread the wash over a larger area. Snake45 suggested sanding the tread; I take that a step further. I'll paint the tread surface with medium-to-light brown, making sure the paint gets in the tread crevices. Then I do like Snake45 suggests by placing the tire on a large Dremel sanding drum, chucking it in drill (Dremels are much too fast IMHO), and sanding at a moderate drill speed.