Hi, first off, I apoligise in advance if my terminology is
wrong. I recently replaced a faucet and drain in my girlfriends
bathroom. Everything was fine, no leaks...except for one.  The
new metal drain body slipped easily into the existing black
plastic waste water drain, and is in there by approx 3 inches. I
used the existing slip joint nut (coloured white) to tighten the
connection between the metal drain body and the plastic waste
pipe.  When I turn the water on, a very small amount seems to
come up the INSIDE of the white slip joint nut and pool around
the top edge of the nut and then comes over the lip , hence the
leak.  I know these nuts are not to be over tightened, Ive tried
undoing and reseating the nut but nothing works. If I put
plumbers putty on the threads on the slip joint nut..would that
fix it, or is that a bad idea..?  Many thanks in advance....