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

What kind of sink.....
a normal bathroom sink with overflow.....or some kind of vessel sink?........

And you said "I've tightened the locknut as far as it could go."......
Is that tight or have you run out of threads?........

My first suggestion.....toss the washer and only use the nut.....

Its a normal bathroom sink with overflow. I replaced the rubber gasket with a new one, and that seems to have fixed the leak. The new rubber gasket fit much tighter on the drain pipe than the one that was inlcuded with the sink. Perhaps the one with the sink became dry or something?