leaky bathrooom sink waste

1 Dec 2007
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United Kingdom
I fixed my waste to the sink with washer in sink but just the big plastic nut below. I even tested it out by filling it with water before I connected it all up - no sign of a leak. Then sods law - when I connected it all up there is no a dribble out os the waste connection. Do I need a washer underneath between the sink and the nut or will silicon do? Do I need silicon in the sink below the trap?
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A bit of silicone sealant or plumbers mait between basin and waste.

Washer between bottom of basin and nut (not too much sealant here, or the washer may squeeze out when tightened).

Then a wipe of sealant round the thread before fitting the nut to stop water running round the thread.
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