Here's my attempt at rusting up the WW2 Jeep from Daz... (click the image for much larger Gallery view)

The rust patches and the dirt on the tyres are procedural blends (using the math blend node) to mix BB's car paint shaders with some scratch / damage color_map nodes, that are driving both color and negative displacement in the rust patches.

The blending is driven by a procedural spots node.

Still working on this... e.g. per Sam's suggestion in the comments I'm taking down the gloss and trying to get the effect of lots of tiny scratches / scours / abrasions on the paintwork.

I thought the displaced, erosion effect turned out looking okay in places at least...

e.g. on the radiator grill and, in particular, on the inside rims of the wheels.