To say I don't know every square inch of my car would be false now. I found a small spot of surface rust on my door bottom and decided it was time to tackle this before it got worse. Luckily I got it before it got bad, just some surface rust with some minor pitting in a few areas on the inner door bottoms. I removed what rust was there and applied some por15 so I'll never have this problem again. I also removed the mtech skirt to check for more rust and found none, this car is clean.
This is a common area for water to collect so check your e34 if you haven't already. The newest E34 is nearly 25 years old so that's 25 years of rain, car washes, and other contaminates getting in there.
I still have to do the driver side. I plan on getting the car painted sooner rather than later now so I'll have to live with the black por15 on the inside for a little while. I get to keep my original doors so it's worth it.