Threads dont seal. Not supposed to. The flare seals. If i do anything with inverted flare fittings is a little anti-sieze between nut and tube.
I snug, loosen and repeat a couple times to seat the flare.
i repaired a damaged line today, guy before me did only a single flare. Bad idea.
I buy all my brake line in 25 foot lengths and make my own. Having the hydraulic flaring kit sure helps.