I've just installed a bathroom sink, and noticed a small leak coming from joint between the rubber gasket and the metal washer. The seal between the rubber gasket and the sink seems watertight. So I'm puzzled to why the joint between the rubber gasket and the metal washer is leaking. I suspect, the water is coming from the "overflow" gaps in the pipe, running in the sink, and leaking between the rubber gasket and the drain pipe and finding its way between the rubber gasket and the metal washer. I followed the manufacturers instructions. Do I need to place some pipe dope between the washer and the rubber gasket or on the threaded pipe? or between the drain pipe and the rubber gasket? I've tightened the locknut as far as it could go. Shouldn't this combination of rubber gasket, metal washer and locknut provide a watertight seal. Do I need to place more pipe dope or silicone there?

Angelo

Replaced rubber gasket