Most of the air lines I've seen are black steel. YMMV.
As I've said several times before, taping or doping threads makes the joints
easier to disassemble. There's no point at all, obviously, in putting it along
the entire length of the pipe. But then, you're the only one who's suggested
doing that. <g>
I said you "could" do it, and "suggested" it would have the same value
as doping the threads in a brass compression fitting. I never suggested
one should do it. I understand pipe dope might make a threaded
steel/iron joint easier to loosen, but not in brass, brass doesn't rust,
doesn't need pipe dope to make it easier to disassemble.
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Or gain? If a compression fitting leaks, "lubricant" on the threads
won't do a thing to help contain the leak.
In fact, since a compression fitting can be overtightened, with
adverse effects, lubricant on the threads may be contra-indicated.
I've never encountered a compression fitting (or a flare fitting, for
that matter) where sealed or lubricated threads would be any benefit
either in assembly or disassembly.
Here, I assume we are talking about compression fittings
(http://www.plumbingsupply.com/compress.html ) not tapered pipe threads
which seal by interference fit (compression) between the male and
female threads. Another post in this thread seems to blur the
distinction between those two categories.
You've evidently not had the pleasant experience of ever having to open
ten-year-old flare fittings on a propane line in a basement with a
dirt-and-cinder floor. Humidity + cinders = slightly acid environment corrosion; I believe that the job would have been a bit easier, had the
threads been doped when they were put together. Yes, I realize those aren't
the same as compression fittings, but the point is that brass *can* corrode,
and if it does, it *can* be much harder to disassemble than one would wish.
Doping the threads before assembly avoids that problem.
Ah, you mean Teflon tape. Plumbers tape (for us old farts) refers to
the metal strapping used to hang pipe.
Teflon tape or dope never hurts (well, for NatGas, use tape/dope designed
for the purpose (yellow tape)).
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