I am installing shut off values and flexible hoses for the hot and cold supply lines in a cabin. The plumbing is all 1/2 inch PVC, so I have a union to go from PVC to copper which screws into the shut off valve. I was told to use Teflon tape, which I did, but it's still leaking. Tape is wrapped the correct way and we have tightened as much as possible.
Is it necessary to use a rubber washer (not sure of the right term) between them to help with the seal? Is that what we are missing?

Thanks

Here's what I do. I start the teflon tape at the end furthest away from the front of the thread, wind towards the end that threads into the fitting, then wind back towards where I started and cross over my end. This prevents any end from catching on the threads I'm inserting into. Then I use a little teflon pipe dope on the outside of the teflon tape to allow the tape to slide in the threads without slipping. This procedure virtually guarantees you won't have a leak.
Good Luck

Raymond VinZant Plumbing Prof.