Copper 1/2" MIP Threads Likely Stripped, Small Leak - But Sweated Fitting

1-2 wraps of teflon tape, then use pipe dope. You can't go wrong. Too many wraps of teflon only distort threads.

Same theory as your idea of epoxy, but more 'plumbery'.

I would remove the valve and replace it with a new valve since the valve you have may have become distorted due to too much teflon tape and over-tightening. And as others have said the valve could also be cracked or have a hair-line split. I would use RTV silicone caulking to seal the threads (costs about $5.00), Install the new valve, tighten and let sit overnight. One word of caution; apply the silicone or any pipe dope only to "male" threads. Never dope "female" threads. I would not try to resolder fittings in a wall unless you have the fire company standing by. If you feel you need to replace soldered joints call for professional help.

Next thing to try is unscrew the valve, remove the Teflon tape and use a good quality pipe dope. Put some dope on the female threads as well as the male. This might fix the leak, but even if it still seeps a little, the seep may gradually seal over a week.

It's hard to tell from the photo, but you should have 1 1/2 to 2 threads showing once the joint is properly tightened. If you have more, the joint is not tight enough and will likely leak. If you have less, the joint is too tight and you may damage the female threaded fitting.

You didn't mention how tight you tightened the valve, so it's hard to say if you tightened it enough, or too much. If you really cranked it down, you might have damaged the valve, the threaded fitting, or both.

If the valve isn't tight enough, try tightening it a bit before trying anything else. If you see the proper number of threads, you'll have to look for another problem.

As others have mentioned, you only need a couple wraps of tape dope. As I've said many times in the past, dope is there to reduce friction, not to seal the threads. More dope, doesn't mean more better.

Remove the valve, and closely inspect the threads on the copper fitting. Copper is a soft metal, and the threads are likely slightly deformed, This deformation either occurred when the old fitting was originally installed, or when you installed the new valve. If the threads are damaged, you'll likely have to replace the fitting.

If the threads of the fitting in the wall don't look too bad, you might still be able to get the joint to seal. I'd probably start with a new valve, a couple wraps of tape dope, and tightening the valve down enough. If you still have a drip, try giving the valve another 1/8th of a turn.

Use nylog and then use Teflon tape over it. Nylog is a sealent so perhaps that would helped