When I took the dash out I had some rust on the inside of it. I went to O'Riley's and got a spray can of a recommended rust prohibitor. Sprayed it on and left it overnight. Area turned a dark blue both. It both treats the rust already there and helps prevent future rusting. Can't remember the name came in a blue can.
I have been using the Rustoleum stuff. Just looks like black primer. but that's only good for thin/surface rust. We are talking chunky scale in these things. lol