Airbrushing Tank Tracks

I don't even dirty my airbrush for tracks. Slop on a base coat (I use a gunmetal or metallic grey). After drying apply a heavy black wash. Apply a lighter wash of rust. Drybrush "steel" onto the wear areas.
Best-o-luck!

Airbrush your base coat on...dry brush with rust, then steel to highlight surfaces. I then rub pencil lead on any metal portions of the track. Dirty them up like the rest of the model. Hope this helps.