Hey, Rigger
I've always given them a basecoat of flat black, followed by a drybushing of metal. I've never tried to give them a rusty appearance because, depending upon the climate, it would take forever and a day for them to rust. Not to mention, they're probably being continually used for this, that and the other thing. Military vehicles operating in the field require a great deal of daily maintenance, especially tracked vehicles.
However, they will collect plenty of dust if they're attached to the outside of the vehicle. So, I usually drybrush a bit of earth brown, again, depending on the theatre of operations, or apply a liberal amount of the appropriate color of pastel chalk thats been ground into a powder.
Peace and Love,
DioRandy