the only way I can think of is rust converter.
If you use converter, DO NOT get the spray ones, they fucking SUCK.
Use the milky type that comes in a bottle and you have to brush it on. Shake the shit out of it.
The black bottle mentionned earlier is this :
They changed the packaging and it looks like this now :
Carquest has them
If your door has rust bubbles, you will have to use a brush on a drill or any way to expose the inner problem, otherwise you are simply wasting your time and money and it'll get worse.
Open up the bubbles, soak the shit out of everything, wait an hour, come back for another coat, then another one an hour later. Then spray a bunch of enamel primer in there and it should be okay for a while. I would then cover the holes with something to avoid more water coming in. Maybe a small piece of duct tape or something.
Also : When using it, never ever dip your brush directly in the bottle, you will ruin the product. Pour some in a small container and work from that. The product starts working almost right away when you brush it on the rust and if you carry rust or used product from your surface to the bottle, it will react in the bottle and will just turn solid on you over time.
A friend of mine ruined a 100$ metal medic jug doing that.