Sealing a bottle trap in a sink

Can someone tell me what the correct technique is? I originally had plumbers mait around the thread / nut under the basin and also under the lip that is on top of the basin. Everything was watertight. However I had to loosen the sink for tiling and since then can't get it sealed again.
I posted on here about how to apply plumbers mait and was advised to use silicon instead. I cleaned every thing up and applied silicon under the lip, tightened every thing up and left it overnight. But it still leaks down through the thread.
It is driving me nuts. I must have given it about 5 goes now, between the plumbers mait and silicone.
Help!!!!
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On 21 Sep,

If it has an integral overflow you'll need to seal the threads/backnut as well.
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