On those impossible threaded fittings, I use both Teflon tape and pipe dope. Chase the threads to make sure they are clean and the right shape slash taper. Wrap the male fitting clockwise and then butter over the whole tape with lots of dope. The cheap stuff like toothpaste works 99 percent as well as the white stuff with PTFE. In a pinch, I might use the permatex copper spray-a-gasket gloop, but it sets up really fast.
In low or non pressure stuff (like dwv lines), I have used seal-all (aka airplane glue) and it has worked where other stuff wouldn't.
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