The conflict between Circolo and jjhepburn arises here because we in NZ simply don't have access to new doors, cheap or otherwise
This is because the rusting was much slower than in the UK and typically took at least 15 years to become a problem, so nobody bothered to stock replacement doors.
To fix properly (i.e. by welding) is actually quite difficult due to the thin-ness of the bottom lip and the risk of distortion to the large, flat door skin. Also the welding tends to leave tiny gaps that lead to more rust. Welding something over the top of rust is also a sure way to continue the rusting underneath - and, the repair will stand proud of the sill line, making it really obvious. On the other hand, welding on more steel tends to increase the strength of the door.
The bog approach, with some etch primer and/or rust converter, holds the rust at bay for at least a couple more years and ultimately, you have to be realistic about the life-span of the car and the cosmetic effect of a few rust bubbles.
If the doors are only mildly-bad, this is the approach I'd take. You can do a couple of bog-repairs (i.e. at least four years!) before the door becomes too much of a mess to work with or the rust endangers the strength of the door. When this point is reached, you should be able to find doors in wrecker's yards that, while not rust-free, are an improvement. Be careful of taking on someone else's inferior bog work, though
Check for evidence of previous bogging - not everyone is as careful - 'pink' bog used straight over rust will not be very successful. Most panelbeaters still believe that filler only sticks to bare steel - this simply isn't the case and leads to faster rusting.
So, to clarify the steps of the best bog-job, use a really aggressive wire brush (e.g. a wire wheel on an angle-grinder) to clear out all the rust holes from the front, and the back where possible, including any previous filler. Dig out all the hardened seam sealer and rust flakes (most will just lift out with a screwdriver from inside the door). Better to remove more steel if it gets rid of more rust. If you can wire-brush everything to shiny steel, just use etch-primer at that point (otherwise, use some rust converter as mentioned). The white aerosol type of etch-primer seems to work really well for us. Then use fibre-reinforced bog (e.g. 'Newtech') to fill the now-primed rust holes and finally sand carefully with a long board, re-etch prime, and then fill with standard filler. Finish up with proper primer and paint, even inside the door, and you'll greatly slow down the rusting.
We found that 'Tergo' rust converter (organic white liquid that turns black) is a waste of time, seems to still allow rusting underneath, and that acid-based rust converters, while effective, need rinsing off with water - which tends to be a problem for obvious reasons! So I'm personally not very fond of rust converters any more, I prefer to remove as much rust as possible with a wire brush, and then etch-prime.
If you did manage to get rid of all the rust, there's no reason why such a repair shouldn't last for a number of years, which is a long time relative to the day or two that it takes to do the job.
To answer thepottleflump's question, to my knowledge, zinc primer does not corrode preferentially because it's not really electrically-bound to the steel (the organic components would interfere with that). Etch primer is basically a high-adhesion primer to ensure the steel is protected from moisture, and it seems to work.
-Alex
