Ouch, that's a tricky one... we have several proprietary rust treatments available here in the states, and I've used several with pretty good results. The trick is to eliminate the source of the moisture and then seal the damaged area with a good overcoat of something to keep the rust from returning. POR-15 is probably the gold standard in this part of the world, and requires several steps to get the best possible results. Also pricey. Now- having just restored my '64 (from Phoenix, Arizona- about as dry a clime as you can get!) with exactly the same problem, can I make a few suggestions? Check the inside bottom of the door. Old VW's had some kind of noise dampener glued to the inside of the door. After decades it usually gets wet, rotted, and falls to the bottom of the door, blocking the drain holes in the area, and holding moisture like a sponge. Result? Small rust-throughs that don't look like much until you start digging into them, and then they become mini-Grand Canyons. I had to have new metal welded in to replace the rotted-out sheet metal, when only pin-holes originally showed from the outside. My guess is that your damage didn't just show up after one exposure to rain, but has been insidiously happening for decades. Check all your door seals and inner and outer scrapers. SOME water will ALWAYS get by them, thus the need to keep the drain holes open. Replace them if they are dried out or missing. Be prepared to find more than you think when you get into the door. If you just try to quick-fix it inside and out, it will come back again and be worse.
Good Luck!
John Scribner