It is Teflon tape and keeps threaded joints from leaking as well as corroding making disassembly easier.
You should replace: -The straight pipe that comes down from the sink (unlettered) -The J-bend (Letter E) -The washer (Letter D) That will give you all clean parts for that washer to seal against. Make sure you install Washer D with the taper side down (it nests in the J Bend.) You need to tighten the nut that goes on top of the washer just snug, since it is a compression fitting...the tighter you draw the nut, the harder it squeezes around that straight pipe coming down to give you your seal. You can over-tighten it and cause it to leak, though...but you gotta really try hard. Just snug it up and if it leaks, tighten it in small increments. You do not need to tape it since it's not a pressure line. The plumber may have put tape on it because he did not have the new parts in his truck, but your leak is most likely between the washer and the straight pipe pointing down. The reason that drain coming down from the sink slips inside of the J-Bend and is held with the compression washer is for adjustment. There's no way to install a drain and then install the sink of your choice and have the distances be exacting, so you got the entire length of that tailpiece for adjustment.
Ok, I must confess Ms. Handy woman was faced with a kitchen sink refusing to drain this very morning. Thinking I absentmindedly had thrown in some old limes and forgot to grind them, I hit the disposal switch. After watching my sink flex under the torque of a 1/3 HP high RPM disposer and realizing the sound was one of serious struggle, I hit the switch to off, squeezed my fingers in, and retrieved my missing dishrag now looking like a cat mauled ball of yarn. Fearing I may have loosened the trap points, I filled the sink, drained it, and checked for leaks. Another big shout-out for Rectorseal #5 as none were noted.
Rub it in Alford, rub it in! I do live in the Eastern part that is still threatening to be part of Idaho. We are gun-slinging hardcore conservatives out here Al!
seems like it's just gonna be the nature of the beast....my husband s fixed it numerous times..I just keep a pan down under
ha!...he has to work on everything...when it breaks or before...poor lads taking a much needed nap now
Update; The leak is fixed, yeah! at least for now. I basically just wrapped plumbers tape around the end of the slip joint until there was enough that held the cap in place so it wouldn't slip down. I think I'll write a journal on all my DIY repairs to this house, lol.
Good job Von and that Teflon tape does wonders. Glad to hear you haven't gone over the edge yet and resorted to Rectorseal.