This is the first time I've used a product that is new to me, Vallejo ModelEffects Dry Rust, which is a dark brown, somewhat granular formula. You can paint it on as a liquid, in which case it dries to a somewhat blotchy paint-like finish, or you can let it dry out a bit and apply it as more of a paste, in which case it dries to a crumbly finish and oxidizes to a range of rusty colours.
I've used it here for all the chipping effects, as well as on and around the exhausts and the hull sides in behind the running gear. It's gone on to the tracks too, over a coat of Vallejo ModelColor Oily Steel, and then been slightly burnished off with a stiff brush.
I like the way it works, and I think I'll be using it a lot. I assume that there's a companion "new rust" version as well; I'll be looking out for it.