I swear by rectoseal 5 for anything threaded or anything I suspect might not fit perfect.
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It’s a fairly thick pipe dope, and I believe it actually has a fibrous content. I always put it on thick, on the male and on the female, and then snug it up pretty good. The hydraulic action on the pipe dope forces it into every nook and cranny on a microscopic scale, pushing the fibrous pieces into any possible gaps.
I’ve used it on virtually every piping material, I use it on flare fittings, etc., etc.
I know it’s a colossal pain in the ass to pull it apart and re-dope at all, but you’ll only have to do it once.
Hope it helps