Fitted new waste outlet to bathroom sink, (the sink has an internal overflow) the inside sink piece has rubber seal (and I've put a bit of extra sealant). Below sink piece has a rubber washer (one side flat and one side domed) and a large nut.
Doesn't seem to make a difference which way below sink washer is put on, seal is watertight. However, water is running down the thread (definitely - I can actually see it).
Have tried white 'plumbers tape' wrapped around the thread but makes no difference, water still forcing it's way out from there.