I have been using rust-mort for decades. It works especially well, if I have a pitted rust spot that I don't want to grind until the rust is completely gone, because removes too much metal. So I lightly grind the spot and treat the pitted area with rust-mort. It neutralizes the rust and I can put primer over it.
Always pour the rust-mort you are going to use into a separate small container before you dip your brush in it. You don't want to put contaminates back in the bottle.