"Restore" Rust Converter is what I use. You gotta wash out the trapped debris that is creating the moisture inside the door & pinch welds of the lower door edge. Water to wash it out. Then regular ol Vinegar to clean it, then flush with water again and let it drip dry and try to dry it with compressed air. Then you have to clean/sand the areas you can reach, coat the visible and insides with Restore or Ospho, or any other rust converter. After a couple of days to fully cure, you can brush over it with POR-15, or KBS Coatings, or even paint. Just make sure that you fully encapsulate the corrosion affected areas to prevent any moisture from getting back into the areas.
Nothing will fully stop the rust except a total removal of it, which in most cases like yours just isn't an option. It's like cancer.