Mud in tracks

Acrylic modeling gel, available at art supply stores in various viscosities and either gloss or matt finish works well for this. Just mix some gel with some light brown acrylic paint, some sand, maybe some static grass if you like, and stipple it on. when dry, dry-brush with tan, buff, or light yellows, then apply a dark brown wash over this to deepen the shadows. At least thats how I do it.

For light dust and acumilated dirt on long convoy marches from blowing dust, I dry brush the bottom areas with a light tan. It worked well on a t-34 I did (gotta get some pics up sometime) but that only really works well for blowing dust that settles on the tank on long marches. I've done this same effect with ground up artist chalk pastels and it really gives a texture to your accumulated dust-dirt, but it doesn't give the look you will get from drybrushing. For huge mud clumps like the tank just went through a muddy area, I use squadron putty and texture it over the treads and appropriate road wheels and what not. I'd then paint the squadron and treads in a regular brown and maybe gloss it up a bit depending if I want to make it look like it just forded a muddy river or something. For a light dusting of dirt on your treads I'd recomend that you paint your treads tanish and highlight the tread raised details with gunmetal or a no. 2 pencil. For dried mud in tire treads, I'd texture squadron in the grooves and paint that. But most important (I'm sure you know this) look at photos of your tank or vehical and see what was the usuall weathering and where the dirt really accumulated. Hope it helps.

Thanks guys