Here's my engineering thought on pipe thread sealing. First a threaded joint is just a spiral path to a leak. Mechanical locking of the threads is necessary for the physical strength of joining the pipes. Once you get good thread engagement you've got all the strength you need. Teflon tape just makes it easier to get more turns by reducing friction. In a ideal world with perfect threads and pipe tapers it would seal, and seal uniformly along multiple threads. But it doesn't. So we need a seal. Using the engineering design principle of turning something that's bad into something that good .... The bad is the pressure of the water causing a leak. The good is we can use that same pressure to push on a gasket and make an ever tighter seal. The more pressure the better the seal. That's where the RTV comes in. It's a remarkable material, first developed for the space program by GE's chemical engineers. It stays flexible over all sorts of environmental conditions and maintains those characteristics for 50+ years. Room Temperature Vulcanizing (RTV) silicone in the threads is the gasket. Having in in multiple threads makes redundant gaskets. Each thread where water gets past a void, it pushes on the downstream RTV which is flexible and makes a seal.
Bottom line clean the threads so you get good bonding of the RTV to the metal, use the correct temperature range product and will get a water/air tight seal the first time.
BTY, the principle outlined above is why flat plate domestic how water coils will always leak. It's a dumb design. Water pushes on the thickness of the gasket from the inside, nothing restrains it other than the squeezing of the studs/bolts. The gasket dries out and it leaks. A better design would do away with the flat plate gasket and use an O-ring in a grove. Then when the water pushed on the O-ring it makes the seal tighter and tighter. If you select the right O-ring material, e.g. vition, it would not leak and have long life. Domestic coils that thread into 4" or larger pipe threads provide much better seals against leakage.