The best I've used so far is to base coat the intended section in the metal color, apply the upper section color (after the base drys) then take Thinner mixed with water and slowly imitate the direction in which the chipping occurs. Practice before hand on a old model to perfect it, but its the easiest option out there.
Thank you for combining everything into one thread, it saves me from explaining in 3 other threads procedures, I also use google a lot to check up on new ways of doing things, maybe try a google search for the basic technique and then inquire to if anyone uses it and how effect it is. Everyone has a diffrent method that works for them.