plenty of chemicals to stop or reduce the rust.. But to actually repair it is another story. Once rust has gotten to a metal part you almost need to remove the piece all together and a few inches where any visible rust is. to do this right you need to replace the door. you do have the option to try to cut it all out and try welding another panel on but many doors and curves and bends that make it pretty hard ro replicate. so I would say go to a salvage yard and HOPE you can find a door that has just surface rust to grind out or one that is rust free..

Auto parts stores and paint stores sell rust converter products in liquid form or sprays in some cases which can be applied to areas to stop the rusting process/oxidation by converting it...iron oxide... chemically into an inert chemical ...or magnetite. Wire brush or sand/remove as much loose flaky or surface rust as possible and apply the converter to any rusty areas...outside and inside the door bottom. Pour or spray a liberal amount on the rusty areas inside the door to coat it as much as possible and get into any nooks or cracks.

The goal is to completely cover all rusted areas so that air and moisture cannot get into the metal again and keep the rust process/oxidation going. Most converter products can also be painted over after they dry so you can use touchup paint or spray touchup to make the outside areas less conspicuous if needed although they may not be sanded too much to smooth them out...the areas may be somewhat rough.

One good product brand is called POR-15...sold online or at some stores which stops the rust process as long as you completely coat all areas...and can be painted over.

http://www.eastwood.com/eastwood-rust-co...

http://www.homedepot.com/p/Corroseal-Rus...

http://www.por15.com/

There's a product made for that purpose called cavity coat or something similar made by several manufacturers. The only one I' have experience with is called "cavity coater" and sold by KBS coatings. Find it at www.KBScoatings.com. I don't have any connection to the company but have used their products with good results.

i fixed a couple of doors once and i took them off, popped rivet a long piece of metal along the bottom of the door and then i smeared body putty over the metal, sanded it down, primer it off and painted it ! after i got the metal on i drilled 2 holes in the bottom of the doors for water to run out ! you might take a shop vac and get the rust scales out of the inside of the door and then spray rubberize undercoating on the inside of the door !

Acid dipping

touch up paint

My trucks driver side door is rusting on the very bottom on the outside and inside along the weld line. However I took off my panel and looked inside to see rust developing inside the door as well.

Is there anyway to treat of remove that? I notice it is building up around the front drain hole and I really can't figure out how I would treat or fix that.

thanks!