This may or may/not be relevant to you, but if you are doing ww2 German desert armour, I learnt this trick from an Aussie model mag.
Paint the vehicle in Panzer grey first. This is how all vehicles arrived in Africa.
Use some liquid latex, like Humbrol's maskol, and virtually dry-brush it on to extremities of the vehicle that are likely to be scratched worn dented or walked on, and make note on a drawing where you did this.
After the latex has cured, spray your topcoats of desert yellow etc and leave to completely cure. (very important)
Then you get someting like a pencil eraser, sticky tape or Blu-tack (not sure about US equivalent, it is sort of a gooey stuff that you stick posters on to walls with) and rub over the latexed spots.
The paint cracks and chips away, quite effectively, and reveals the panzer grey coat underneath!
With a bit of practice, this can be really cool, and can be applied to other areas of armour as well. I went to town on a model wreck to get the hang of it. Less is more with this, just like most weathering techniques.
Quite coincidently, this is quite fun too.