On 27/04/2020 18:56, John wrote:
If only people would realise that it is the gasket that created the seal -
the tread it sher to pull the faces together - and what about a smear of
coppaslip?

While this is true, that doesn't stop limescale deposits forming in the
clearance gaps between the non contact thread faces. My preferred
fitting method on a heater that needs replacing every year or so (long
story) used to be to put a thick layer of PTFE tape on the threads only.
Excess extrudes and keeps the clearances filled, very much reducing the
amount of deposits. More recently I have been using the soft sealants
that claim to be "liquid PTFE" (although they are obviously not), for
the same reason.