John Rumm wrote:

Andy Burns wrote:

I don't routinely use any gunge on compression fittings, but have used
LS-X on problem ones (both copper and plastic waste) to good effect,
it has a very long shelf life.

Yup IME LS-X will fix pretty much any poor joint that would be very
difficult to fix properly.

Difficult in this case was one leg of a radiator pipe that was perhaps
1/16" too short to snug-up as it used to, after laying a wooden floor on
top of the floorboards, didn't want to rip up the floor in order to
extend the pipe with a coupler from below floorboards level.