I've been trying some more weathering techniques, and on this one I was going for a severely oxidized look, as well as the rust and grime. I used a base wash of diluted acrylic (burnt sienna) first, to get rid of the sheen and give the following applications some "tooth" to hang on to. Then applied some chalks (Bragdon Enterprises), followed by Rustall liquid, and finally a couple of coats of Rustall black wash. The oxidized look came from the Rustall liquid - that stuff is very cool stuff indeed. I really like it, and the nice thing is that if you feel it's too oxidized, you can lightly spray as much Dullcoat on it as necessary and the oxidization will fade away.