This is what I use here when working with NPT fittings, whether it be for the house or automotive. Been using it at least 30 years now and tossed out all the teflon tape once I discovered it. I just used some the other day to install a new hot water plate exchanger for the boiler. Never had a single leak since switching to it.......:
https://www.autozone.com/sealants-gl...E&gclsrc=aw.ds
I'd add here that a few years ago my brother who does HVAC, in floor heating, etc was complaining about the poor quality of NPT pipe, fittings, etc and was having leaks on occassion and having to re-thread fittings and pipe due to poor sealing qualities. He was using teflon tape and not always able to get things to seal up which is very frustrating on a big job when you are all done and do your pressure test. I bought him a tub and he stopped using teflon tape and hasn't had any issues since the switch.....
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