I remember back in the late 1960's working in a high pressure ammonia freezer plant. All the pipe threads of the various sized steel pipe were doped with a two part sealant, the first component was a yellow powder known as litharge, this was mixed with glycerine until it became a thick paste which was then applied to the pipe threads. When it dried it became rock hard. When you had to dismantle pipe put together with this dope you had better remember where the 36 or even 48" pipe wrenches were. Any other older guys remember working with this?
dope on the threads is acceptable if not adviseable. dope on the faces never, unless maybe it's 4:59 AND ONLY if all else has failed and is failing. Even then, I would not use it, but I've seen it done.
A bit of oil on the faces of flares and unions never hurt.
Some utility companies recommend a non-teflon dope. I'm not a utility, but because of that non-teflon rumor, I got attached to the NT variety of a brand name pipe dope.
But it doesn't come out of your clothing...EVER!
I also dope the threads of a union. Not for the sealant, but to lubricate the threads. I to at one time laughed at the plumber that doped threads untill I realized that those are the ones that I didn't need a hammer to loosen.