Our hardworking Curator @davesaddiction suggested this idea. I had a Goodwill find Greenlight '74 VW T2 Campmobile on hand that I'd already lowered the pop top on, so decided to use this as the "mark" for this how-to.
Start with a fresh casting or one that's customized already.
Here are your main media. Diluted grey (for road grime) and brown oxide (for rust/mud), I used "Apple Barrel" brand from Walmart, but Vallejo Model Color from a hobby store or Hobby Lobby is good too, and Tamiya black panel line wash for grease, gas tank drips, engines, mildew, etc.
Start by using alternating large soft and rough brushes to put on a mixture of grey diluted paint and a little brown oxide. I used Dollar Tree paint palettes to mix.
Let dry 20 minutes or so. Your result should look sort of like this.
For rusting/mud around the edges, carefully dry-brush, with a small hard/rough brush, around panel lines, known rust traps on the real thing, areas you want suggestions of body damage, etc. with some of the Brown Oxide.
Let dry for an hour. Your end result should look like this before drying.
Here's where the Tamiya panel line wash comes in.
Apply the wash, with the little brush inside the lid, to areas where you want mildew, petroleum, soot, etc.
Let dry 30 minutes-1 hour depending on room temperature. Outdoors works too, in summer or if you live in a tropical or desert climate.
This is the end result. Note I got a little ahead of myself and thumb printed the door. This wipes off before drying time completes & can be easily redone.
Now if you want a full-on junkyard/barn find look?
Grind up some earth tone oil/chalk pastels and apply them all over the casting with a very large, soft brush, preferably while paint & wash are still tacky. Let sit for 10 minutes or so.
Use your thumb or a clean used toothbrush to wipe off excess weathering compound.
End result should be something like this.
I will update later on in the next few days with pastel and makeup powder dusting, large scale styrene interior/chassis aging (should be applicable to diecast & resin & white metal with adjustments), and large scale weathering.