Hi, on my '86 420, I noticed recently the lip on the door bottoms which holds the rubber weatherseal in place, was starting to rust underneath the rubber; I suppose it holds moisture, as we don't get salted roads here). I pried the rubber out of the way, cleaned and sanded the rusty spots the best I could, then used Rust Converter. I will spray the spots with some aerosol touchup paint I found which almost matches (it won't sow anyway).
I only found this by accident; fortunately before any serious damage was done. I thought this might help you all as an alert to check this on your cars before it gets too bad.