In article , Jack Stein wrote:
Doug Miller wrote:

Where have you seen air lines with compression rings in the fittings?

About 30 feet from where I'm sitting? Are you suggesting compression
fittings are not used in air lines? I'm pretty certain I'm not the only
one to use a compression fitting on an air, or a gas line.

Not suggesting they're not used, just suggesting they're not common.

I guess we could argue common, not common all day and get no where. I
would guess it depends on what you are connecting together. Sometimes a
compression fitting is common, sometimes not. Air lines commonly have
flexible copper tubing for example and it is very common to use brass
compression fittings where they connect to standard pipe. You would use
pipe dope on the pipe thread, but not on the compression fitting.

Most of the air lines I've seen are black steel. YMMV.

Anyway, if one is using a compression fitting, pipe dope or tape is not
needed, and may in fact be detrimental to the connection. I have seen
compression fittings where pipe dope was used on the joint by someone
who didn't know any better.

As long as you keep it on the threads, and off the ring, where's the harm?

No harm other than a waste of time. Brass compression fittings simply
don't need dope on the threads. You could put pipe dope along the
entire length of pipe and do no harm, but it begs the question, why?

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