I read through about eight pages of this, found the fix instructions and concur with the approach that involves the proper application of POR 15. I have used that in some very extreme applications (mild steel on Jeep used in very harsh environments) and found it to be the best solution for me.

There is a material that body shops use to re-installed rubber channel seals - it's a "release agent" that makes the rubber slippery and allows it to be stuffed into the channel easily. It evaporates without a trace. I keep a can (spray) on hand for these types of jobs. Sometimes I'd be lost without it. You do have to be careful not to get it all over the place but it's great stuff.

A few comments, for what it's worth:


Undercoat - fails and actually makes matters worse - this is a big mistake - years of experience here - undercoat traps the moisture that's there, traps new moisture (it develops pin holes over time), conceals the problem, and because of this becomes a petri dish for new rust. Don't do it!

Waxoyl - was my favorite rust preventative (I had Jags) until I learned it must be reapplied often because it eventually rinses away over time. It's great stuff to put inside doors in the seams where it won't wash out and can protect the seam from water getting in, but not so good for this type of problem.

spray-on rust "converters" - the active ingredient in these actually is what forms the first step in the POR-15 treatment. It is "OK" but not a long-term, permanent solution. And they interfere with a POR-15 treatment because they have paint mixed with the active ingredient which interferes with proper adhesion of POR-15.

cutting off the weather strip - I'm not sure why to cut it off when it can be removed cleanly and reinstalled later with no damage. Also think it compromises the seal which may lead to other problems.

This area on cars is historically one of the most common rust problems. That suggests manufacturers would by now have turned the corner on a design that would prevent it. But realistically an opening at the bottom of the door is the last place moisture will land, and if it gets trapped, it's going to cause rust - unless it's on a non-metal surface.

Treat the rust properly and early, and periodically inspect the area for new rust.