Hey, this is a basic breakdown of how i do it:
Paint the surface normally, get a sponge, dab it in some dark brown paint, then test it on some paper, so it looks irregular, but not too fine.
Then dab the sponge (or blister foam) onto the mini in the places where the most wear and tear would occur (edges). After this, take a fine detail brush and \"underline\" each chip with a light coloured paint.
After this you can chose to have bare metal showing underneath, i only do this in the largest of chips. Like with the underlining, take a fine brush and paint boltgun metal into the chips.
Finally add rust, or whatever effect you like.
Alex