I just installed a hot water expansion tank, as my first ever "real" plumbing job, put everything back together, and ended up with a bit of seepage after restoring pressure to the system. (I learned how to solder copper pipe from YouTube and from some of your posts, and that part is fine.) I also flushed the water heater a few months ago, and replaced an ugly anode rod with a new magnesium rod. The water heater had a bunch of sediment, but no sign of rust to my untrained eye. But that's mostly beside the point. Incidentally, this is a 66 gallon AO Smith EES 913, possibly dating back to 93, judging by the serial number.
My main problem seems to be with the threaded female inlet to the tank. It looked ugly to start with, but it wasn't leaking.
After understanding what I was looking at (I think), I found myself a little dismayed at how few threads were there and at a bunch of black gunk and slightly charred dip tube. After cleaning all of that out, I took my new dielectric nipple, put a bit of teflon tape on the ends, tightened it with a wrench, screwed on my copper contraption and soldered it to the water line, and opened the valve first slowly and then all the way.
Even before the system was pressurized, and only exposed to the full stream of water coming in, I started seeing seepage at the top of the dielectric nipple. After consulting google, I cut my pipe, unscrewed everything from the nipple, cleaned the water inlet threads with a wire brush, and applied four wraps of teflon tape in the right direction, with pipe dope on top of it. I tightened everything down really well, giving it a full turn more than intended because of the angles that I needed to preserve to mate with the water line and my expansion tank mount. I soldered everything back in place, and there was no more seepage at the top of the dielectric nipple. I let the tank fill completely with all hot water faucets off, and everything seemed to be okay until I noticed the tiniest little drop of water poking through the pipe dope at the base of the dielectric nipple.
All of you will know that it's difficult to tighten anything once it's hardpiped in place, but I really didn't want to cut the whole thing out again, and I was pretty confident that I had applied the tape and dope correctly, and had tightened things sufficiently. Nonetheless, I heated up the tee fitting just above my threaded female adapter, and forced most or all of another turn with an 18" wrench while the solder was molten. I turned everything back on, and found the same seepage.
Now the water header is quite old, but I've seen no problems with it in the year-and-a-half that I've owned the place, and I'd rather not replace it just because of a problem with a fitting. (You will be amused to hear that I decided to do all of this myself because I wanted to save some money!) So I thought to myself, why not use JB WaterWeld or JB MarineWeld? I could try to sand down the top of the water inlet fitting and permanently glue/weld that to the dielectric nipple. The pro is that I wouldn't have to cut out my pipe and rebuild it yet again. The con is that access is poor (the brown cover isn't centered over the fittings), and I'm not positive I would be able to clean the surface sufficiently. But another option that occurred to me was to use Loctite (the red permanent stuff). I would have to cut my pipe out, and I would be attaching the nipple permanently to the tank, but if it got rid of the seepage, I'd be satisfied.
By the way, my hat is off to your profession and the crazy things you have to deal with.
Any thoughts on what I can do about this? I think my problem stems from an old and damaged fitting on the tank, so I probably wouldn't experience this same thing on a new water heater, but I'll admit that I'm now far less confident about these jobs.