Recently did some re-plumbing and installed a union. I made sure everything was straight and lined up and tight. There is very minor leak between the threaded nut and the fitting that it sits on. More specifically, it leaks from the hole in the nut that the fitting passes through. I used Teflon tape on the threads-they are dry. The nut threads on very easily indicating that everything appears lined up.

Do I have to refit the union, or can I put some Teflon sealer between the nut, around the hole, and the fitting to seal it?

Recently did some re-plumbing and installed a union. I made sure everything was straight and lined up and tight. There is very minor leak between the threaded nut and the fitting that it sits on. More specifically, it leaks from the hole in the nut that the fitting passes through. I used Teflon tape on the threads-they are dry. The nut threads on very easily indicating that everything appears lined up.

Do I have to refit the union, or can I put some Teflon sealer between the nut, around the hole, and the fitting to seal it?

If this is a ground joint union, (see image) and a leak develops I coat the ground faces lightly with sealant. Good luck, Tom

Tom, thanks. It's for copper pipe. The fittings obviously copper, but the couplings are cast copper (brass?) and are ground/machined. Will add a dab of sealer...

In addition to Tom's advice: Use two medium size pipe wrenches when tightening the union. It is metal on metal connection so apply good pressure.