Someone help.
I've been working on installing a new sink for almost a week now.
I've tried multiple configurations with multiple adjustments to tightness and connections. My lavatory is still leaking when the faucet is turned higher than about mid-pressure.

The leak originates from between the bottom of the lavatory and the gasket/locking nut underneath the hole.

I have secured the seat into the pop-up assembly with Sta-Put Ultra (which states it is perfect for all plumber's putty applications). I go to tighten the assembly in the seat while holding everything in place. The putty is pushed out from the drain and as I tighten the locking (mounting) nut I keep the drain body stationary. I crank down pretty hard on the nut to get it as tight as possible. Everything seems good, no leaks in the extension connection or in the p-trap. Leak continues to come from the bottom of the underside of the lavatory. I have teflon tape on the threads and have run out of patience for the leak continuing. This is a pedestal sink, I have no idea what else to try. Any help or guidance in appreciated. Thanks.

Ryan

Start fresh.

Clean all surfaces.

Examine the sing and the underside of the pop up (sealing surface) for burrs. Look for small cracks in the sink.

If OK, then roll a length of plumber's putty to about 1/4" diameter.

Just place it on the underside of the top sealing surface.

Insert into drain.

The rubber washer goes on next, then the nut.

DO NOT use teflon tape on these threads. The only useful stuff is plumers silicone grease. Yep, grease. It just makes things easier to remove 10 years later.

Finger tight first. Make sure it's plumb and tighten till the goop comes out. Keep removing the excess putty as you tighten.

There is no reason to overtighten. You should need no more than 1/2 of a turn when it feels tight.

Install the rest of the stuff and wipe off the bottom of the sink.

Tape some toilet paper above the drain at the bottom.

Do your leak test. Want to make sure the leak is coming where you think it is coming from.

If it is a plastic pop assembly then I feel your pain. They are very difficult and I usually talk my customers into springing for a metal pop up assembly.(Only come across these on customer supplied faucets from the box stores). The plastic ones are listed under JUNK in my book.

Agree with CHayn, get a new drain assembly. Went through 3 on the last lavatory installed. I think the problem lies more with the Chinese made lavatories.