How long do you leave the salt on the rust primer before you paint another color and then (no and then) how long does the salt stay on before you brush off to reveal the rust primer.
Minutes: I just wait until the water dries.
I'm experimenting with when to take it off. If you wait until the paint is dry, you get a very flat look. If you wait until the paint dries just enough (looks waxy), you can brush it off and create blisters.