I have an old 10 gallon Polar Ware pot. I clean it with Bar Keepers Friend after each use and let it air dry. I wipe it out with a paper towel afterward, always has black on it. I believe it is oxidation.
I have always naturally carbonated all my kegged beers. I brew 6 gallon batches, bottle 3 gallons and keg 3 gallons. I prime the entire 6 gallons like I am going to bottle the whole thing. I purge the keg 3 or 4 times at 30 psi and seal the keg at the same pressure. I have never had a problem...
If you're still have a problem with leaks, you might want to try this:
https://www.homedepot.com/p/1-2-in-x-260-in-Yellow-Thread-Sealing-PTFE-Plumber-s-Tape-31403D/202078116
I've used it on my manifolds and haven't had any leaks.
I use these:
https://www.officedepot.com/a/products/837378/Office-Depot-Brand-Removable-Round-Color/
I print the bottling date, beer style and ABV and put them on the cap.