More shortcutting than anything. The rusty metal is just whatever flavor of darkish metal you like (I used Vallejo Gungrey) put down first, then followed up with a mix of Skin Wash and Rust; about a 1:2 ratio
Th model air Rust is odd. It's terrible for normal coverage,and yet too thick to be translucent. I thinned it with water but it just got blotchy. By thinning just a bit with skin wash instead it works as a nice glaze, knocking down the reflectiveness of the metal but not covering it entirely. I'd bet mixing in a little rust pigment would crank it to 11, but on normal boyz it's not worth the effort.
I will say this effect looks best to me on surfaces that get a lot of wear but get that rusty sheen, like monkey bars at a playground. The dangerous, unsafe awesome ones that most of use grew up with. It has an almost polished rusty quality from a generation of hands.
For hardcore rust on vehicles I do it different to get more texture.