In article , "Roy Hammond"
says...
What waterproofing product would you recommend for applying to plaster on
either side of am internal breeze-block wall ?
Problem is the bathroom shower mixer connections have leaked very very
slowly over months if not years - only showing evidence when the wall on the
other side (in the hallway) started to lift and flake plaster. Hopefully the
miniscule leak is now fixed, but want to be sure that any moisture remaining
(or recurring) in the wall doesn't cause more problems.

Best way to do that is leave it to dry out.