What I also heard is that you:
1) primer your piece
2) Spray paint it silver
3) next put Mustard, ketchup, barbeque Sauce or whataver you like on the places you want battle worn, just scoop it on there. ketchup is great because they often come in such bottles with a narrow opening so you can place it well.
4) Paint color of you choice
5) Remove the ketchup/Mustard, BBQ Sauce to find the silver spots underneath
6)roughen the edges of the ketchup spots with the 2. Technique BobvDijk described above