I have been trying my hand at weathering and got pretty good results at rust, sut and grime but I want to give some cars that dusty look that gathers on the bottom half of box cars after some time on the tracks. The look almost looks like a whiteish mist. Could I achieve this without an airbrush? Or is it possible with spray paint? If so what color should I use. I would like to know some techniques for achieving these results.