Tried something a little different this time. I did a magic wash (50/50 Future floor polish and Vallejo Oiled grime) all over the buggies, but it didn't pool the way I wanted. So I washed right over with a little black mixed and while it worked, it tinted the whole model and killed off some of the overall contrast. So I went back to a known technique, I airbrushed straight thinner on the model and used the airbrush to powerwash out from the centers of panel, pushing the relatively thick magic wash to the edges, thus preserving the contrast I put down with my base tone layer. I figure a 2-1 wash/future mix is about right for the initial wash, then airbrush soon after, so the floor wax has a little time to gel. I'll see if I can refine it and make it more repeatable.
Now I wouldn't do this on anything "clean"; Marines, Tau, Eldar, etc wouldn't look right.