Thanks fellas! The exhaust staining was airbrushed in two steps by using heavily thinned brown and brown+black. Using Tamiya Acrylics and iso alcohol as thinner, my paint:thinner ratio is about 1:4... very watery. Since it is so watery, it is essential to use very light (reduced air pressure), repeated strokes to build up the effect. Brown first and then the brown+black in a narrower area to leave a lighter brown edge to the stain.