The salt technique goes like this (for the most part):

Spray your base color

Get some salt. Sea Salt, the larger stuff

You can grind/crush some of it to get a variety in sizes of salt

Moisten your model with water

Put salt on it (basically the salt becomes a mask)

Let it set a bit so the salt will stay put

Spray your next color

With a stiff brush wipe/scrub off the salt revealing the color underneath.

This simulates chipped paint. Could also be used for subtle color variation too if your top color is only slightly different than the base coat.