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