My favorite non-hardening sealer is Permatex Super 300. (Not 3-D) It was developed to seal aviation gasoline fittings, so it's definitely useful on pipe thread fittings for fuel pumps, gas tanks, etc. Flare fittings do not need any sort of sealer, as the threads of the nut don't do any sealing- - - -the angle of the tubing flare and the part of the fitting it meets seals the liquid.

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!