I used a couple of sludge washes with some spatter effects in between.
The spatters were done by loading a brush with thinned oil paint, holding it near the model and hitting it with little bursts from the airbrush.
The dirt gets thicker towards the lower parts of the models. I wanted to simulate the dirt accumulation of a mission that included a long trek across swampy and sandy terrain.
It needs some final touches; some pigments here and there, oil stains (basically high gloss accents around the engines and gas caps and a thin layer of sand/dust).
I think these will just get a simple base, nothing too elaborate.
Thanks for looking.