I have a leaking copper union around my DHW mixing valve. There is a slight blemish on the seal and I CANNOT get the thing to stop leaking. I tried valve lapping compound to try and "smooth" it out. No good. Pipe dope sealants last for about a month or two then start a slow leak that gradually ends up leaking faster. Just thought about opening it up, adding flux and soldering it together. Anyone ever try this with success? If not I'll have to do a lot of plumbing redos. I have grown to hate copper unions.