Salt chipping is a subtractive method of achieving the look of paint that has worn off due to years of being exposed to weather elements. You have to paint a bottom coat that will be exposed by the chipping and then the main color over the top of it. Apply the salt with a wet brush, letting it dry so that it sticks to the top paint layer, and then removing it will achieve the effect.
John Crowdis explains how to he applies his salt chipping technique on his great YouTube channel called JC's Riptrack: https://www.youtube.com/watch?v=qGmiqPBhII4.
I highly recommend checking out the other videos on his channel as it contains a wealth of information.