The slip joint where my PVC sink trap enters the copper drain stack is
STILL leaking.

I've been to the plumbing supply, Mom&Pop's Hardware, Ace, Home Depot,
and Lowes. NOBODY has heard of a "special" slip joint nut and washer
for a copper fitting.

They keep sending me home with exactly what I already have. New trap
parts, nuts and washers slow the leak to varying degrees, but never
stop it.

The water is CLEARLY coming out between the washer and the PVC trap
pipe.

Should I just slather some liquid pipe dope on the PVC, stuff it into
the copper, nut it up, and be done with it?

If that doesn't work I'm going to wrap it with Mighty Fix-it tape and
give up.