I finished this today...
I tried chalks, and didn't really like the results (probably more my lack of talent than the chalks). Since then I've used paints for large scale models, with which I'm fairly satisfied. First you paint the whole model like it was brand new, then paint in the drips, runs and rust spots (a bit brighter than you want them) with a brush.....Then lightly overspray the whole model with brown, grey and black primers. Add a bit extra grey and black from the top to simulate falling ash, and a bit more brown (light tan if you can find some) up from the bottom to simulate kicked up dust. Finally paint on a bit of grimy black around the chimney or exhaust openings with a brush and you are done. It's not award winning weathering for photo contests, but looks pretty good from about 3 feet away and it's fast if you need to do a dozen cars at a time. It also doesn't smear, fingerprint or run.
Thought I'd add a pic...I did these last night.
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