Re: 1" threaded copper union leaking
Only the seat of the union is intended to provide a seal. The threads are not meant to seal; they are only there to provide a mechanically strong connection. The seats should provide a good seal with just metal-to-metal contact; no compound or tape necessary.
If the union is not sealing, then the seats are not mating properly. As bp21901 says, it may be a poor-quality union.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.