it will happen again no matter what. You can sand it down to metal, if there is metal left and it's not just crumbled rust. Then prime and repaint. If it's too bad the rust will need to be cut out and new sheet metal welded in.
Rust sucks and there really isn't anything you can do about it other then move, keep the car in a humidity controlled garage and never drive it, or wash and dry it completely after every time you drive it.
205,125 miles when she was given away
35 mpg last tank
̶w̶a̶i̶t̶i̶n̶g̶ ̶f̶o̶r̶ ̶h̶e̶r̶ ̶t̶o̶ ̶d̶i̶e̶ ̶s̶o̶ ̶I̶ ̶c̶a̶n̶ ̶b̶u̶y̶ ̶a̶ ̶t̶r̶u̶c̶k̶,̶ ̶b̶u̶t̶ ̶s̶h̶e̶ ̶w̶o̶n̶'̶t̶ ̶d̶i̶e̶ She didn't die but I bought the truck anyway!!