If you're making a joint on an *installed* pipe - as the OP is - it is
often difficult to get access to clean it completely, particularly if
it's got paint on it.
A compression joint with a smear of LS-X or - if you prefer - a wrap of
PTFE tape can cover a multitude of sins.
A compression joint shouldn't have any compound or tape on it.
It's designed to seal without anything added to the sealing faces.
(In the case of carrying gas, it *must* be assembled according to
the relevant BS, i.e. without any sealing added.)
However, if it's been buggered up by fitting badly, then the seals
may not work as designed, and can often be bodged by using tape
(again, not acceptable for gas).
This will not seal for as long as a properly made compression joint.
Once the seals have been contaminated with something, it will
probably always need bodging to make it seal again because the
sealing faces are damaged.
The most common way I see compression fittings damaged is by doing
them up using a poorly fitting spanner on the compression ring.
This can distort the compression ring into an oval or egg shape,
and then it won't seal, and may even leak more as it's tightened.
The spanner needs to be a good tight fit so there is no tendancy
to roll over the nut corners. The jaws need to be parallel, and
if using an adjustable spanner, they must stay parallel under
force (cheap adjustables don't). I do sometimes use a single turn
of PTFE on the non-sealing half of the olive as a lubricant while
tightening and compressing the olive on large pipe sizes, but if
you do this, make really sure it doesn't get into the sealing
surface of the joint, and after initial assembly which compresses
the olive, you can disassemble and remove the tape.
If the capping off is temporary, I will often do it using a
pushfit end cap (although they don't like being pushed onto
hacksawed pipe ends with any burr as it can damage the O-ring).
When doing plumbing, it's good to have some pushfit endcaps to
hand anyway in case you have to cap something off you hadn't
thought of (e.g. water trickling back out the other end of the
[email address is not usable -- followup in the newsgroup]
More than likely...
My experience is :
Generally if the pipe is clean and in good condition (not scratched,
dented etc), then a compression fitting will seal just fine on its own.
Using some lubrication on the threads to make tightening easier
(especially if working in a confined space) can make the job easier. A
pipe jointing compound like jet lube will do the trick, but so will a
few turns of PTFE tape on the threads.
If for some reason you can't get a good seal on a joint (maybe 1 in 100
joints IME), then a few turns of PTFE over the olive will normally fix
it. As will application of a jointing compound. (use a proper WRAS
approved one though - not something that will harbour microbial growth
like putty or old school boss white).
lastly a decent set of spanners help enormously. Using something that is
poor fitting can in extreme cases deform the backnut, and that will have
difficulty ever making a proper seal.
 You will hear people bleat on about that being "pointless" and
"amateur" etc, but that is because they are too dumb to understand that
lubrication of the turning bits has a completely separate purpose from
augmentation the sealing surfaces with similar materials.
They are good for thing like plunge cuts into a pipe surrounded by
others. I tend reach for my small 10.8V reciprocating saw for most
plumbing cuts that I can't do with a pipe cutter since its much faster
and the blades are cheaper and more robust.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.