On Mon, 24 Jan 2011 08:44:17 -0800 (PST),
wrote:

The P-trap under my kitchen sink disintegrated the day after
Christmas. Simple DIY repair. No big deal.

I replaced the metal trap with a PVC trap, figuring it wouldn't rust
out or leak ever again...

I can't get the joint where the P-trap enters the copper drain stack
to stop leaking. It's leaking out around the sealing washer, NOT the
threads.

Tightened the nut. Still drips.

Replaced the PVC nut with a metal nut and rubber washer. Still drips.

Do I need to go back to a metal trap?

If you get a good firm fit with the nut, I'd just dope it up where
it's leaking.
There's no pressure, and any waterproof dope that dries flexible will
work.
I used to use something called Liquid Rubber for troublesome drains,
and it always worked easily. Came in a small tube that lasted
forever. Don't see it around now. Probably butyl rubber.
White silicone bathtub caulk works too but it isn't as sticky as the
Liquid Rubber so you need a wider bead, and you should let it cure
before using the drain.

Some of these PVC fittings aren't formed to fit older metal fittings
well, or even other non-matched PVC fittings.
The washers are different thickness and shape depending on what pack
you grab, etc.
Flanges are different thicknesses too.
Just did my concrete basement tub and the PVC tailpiece flange and the
washer combined were way too thick for the nut to grab enough threads,
so I had to go pick up a metal tailpiece.
Didn't have to use dope, but I wouldn't have hesitated.

--Vic