Road or Track Spray
Road or track spray is very similar to dust in the way it reacts when its airborne and also affects the same areas on vehicles. Spray sticks to surfaces a lot better than dust and is much more even toned in colour. I suggest heavy dry brushing or the use of an airbrush to achieve this effect, gradually building up the colour to create the right density. This effect can also be worked back into where things are used or moved as described in the section on dust.

This line up of Tracked Rapiers illustrates how track spray sticks to the same areas on the same type of vehicles. April 1996

This Challenger ARRV has a good covering of track spray, with darker patches where fuel or oil has been spilt, with evidence of soot from exhaust system. May 2004
A Leyland DAF DROPS with a even tone of track spray and a black fuel spill emanating from the filler cap on the fuel tank. July 2006
Copyright Tim Neate