When everything under my bathroom sink fell off, I installed a new metal tailpiece, then went PVC with trap and drain extension, which slips inside a 60+ year old galvanized(?) drain stubout, the problem area. I have tried a variety of washers with different profiles, but they all leak. The closest I got was when I whittled down a washer I had left over from an adapter kit, and had a drip every five seconds. Tightening the nut turned the drip into a stream. What really frustrates me is that I can see that the drainpipe in the wall is lower than the floor of the stubout, so water should want to drain instead of backing up and dripping. Any suggestions, including cheats?
Are the threads still good on your galv. stub?
I have a trick where I use a bastard file to "flatten off" the front end of the stub-out, then run my 1 1/2" pipe die on to chase the threads, then use a rubber slip-joint washer instead of a plastic one.
The reason for using rubber is that the plastic washers rely on an even squish provided by the washer pressing into the i.d. of the old galv. pipe, which is often out-of-round due to age/corrosion. The rubber washer just butts against the end of the galv. pipe and the squish is provided only by the slip-joint nut.
P.S.- slip-joint connections need to be tightened with a wrench. Folks are often mistakenly instructed by Big-box teenager that "hand tight" is sufficient, it is not.