Joe's Model Trains Co.
Joe's Custom Weathering Dyes - 1 Ounce Bottle

Clay Dust
Walthers Part # 350-115, p. 832 Walthers 2012 HO Scale Reference
A scale, $8.45, currently in stock at Walthers

 
Won't harm paint finishes, dries flat and can be sprayed with an airbrush. Offers superior adhesion even to glossy finishes. No sealer coat need. Dye won't rub off when dry but is water or alcohol soluble at any time.