New Delta bathroom faucet i just installed is leaking through the theads of the jamb nutt under the rubber gasket. The directions did not say to use anything on this part of the tailpiece where the jamb nut is.
My guess is the rubber gasket is not sealing the threads. I took apart today and this time wrapped with Teflon tape (the part the rubber gasket slides up and the jamb nuts screws up) as I didn't have any putty or pipe dope on hand. I also redid the top flange with silicone and tightened the jamb nut pretty good with channel locks.

It's sitting for 24 hours for the silicone to cure before I test.

If this leaks again any ideas? My next step would be remove again... scrap off silicone on flange, redo flange with silicone, but this time use putty on top of the jamb washer and in the threads.

Thanks for the advice

 

Depending on the type of sink...... you'd need to use silicone or putty under the flange of the tailpiece.
So that would be on the topside between the tailpiece and the sink.

 

Already did the silicone on the topside under the flange. I'm talking underneath. Water was following the threads down. My guess is water getting in the overflow holes and then making way down the threads between the rubber and compression nut on the underside. It's tightened pretty good with channel locks too. How tight should it be...(it's a metal nut not a hand winged plastic nut)

 

I don't remember ever having used a sealant below the sink.