Rust mort instructions?

[NorCalGal]

Reading the rust mort directions..... it says do not apply to paint or bare metal or it may cause paint adhesion problems. My issue is that I have mostly light rust but there are spots of rust randomly throughout the fender. Do I spray the whole fender the wipe over it with a damp cloth to pick up the excess or what?
When I bought it the guy at the paint store said to spray or brush thinly over the whole panel. I'm having trouble finding the correct information.