Dear Mike Landry and the usual crew,,,,
When installing any "drain tailpiece" under any sink or lavatory bowl it is very important to use the plumbing parts,,,
Usually this involve a nice healthy fresh black rubber washer,a thin brass or nylon "friction ring" and finally a threaded slip nut with the flat section pointed up
A careful application of fresh plumbers putty can help but is usually not need if this operation is done right,,,however ,,,and you wont believe this guys,,,I like to use the old standby Dap caulking in the red and white tube.Sqeeze out more than you need on the contact surfaces,tighten things up ,wipe off excess with a damp terry towel and if you have a "leaker" giving you trouble,let things dry overnight before you run water.
Having said all that I strongly suggest you find find an old school mom and pop hardware with a good plumbing section that offers hands on help or a wholesale plumbing supply that will sell to the general public.Make a friend there and they will show you the correct parts use.You could be getting failure because you are using the wrong item,,,,Nothing is Standard in Plumbing,,,,
Notice a message I'm giving here guys,,,always use new,supple fresh rubber parts when working with drains,brittle aged parts create leaks,,,
An aside and a story about Dap.I had a plumber buddy who had a stray rock hit his gas tank on his truck on the way home from a job .Fortunately he found the small hole leaking gas.Being at a lose what to do stuck out in the middle of no where ,,he cut out a square piece of black neoprene rubber from a no hub coupling ,together a loop of electricians "tie wraps" and got out some "Dap".He applied Dap to the hole,covered it with the rubber patch and tightened up the tie wraps,,,the sucker worked,,no leak!!!
Being a busy plumber,,,he forget about the damn thing till it crossed his mind about 6 months later,,as crawled under the truck to his amazement the patch still held,,of course at that point he had the mechanic drain the gas tank and repair the leak.That is the power and "glory" of "Dap",