How tight should a 1in copper union be tightened?
I have 2 that I have tried to tighten to no avail.
Still very small leak under air pressure test of 40psi.
I did sweat a threaded adapter to the union.
The seal looks clean but dis-colored.
Is there a way to lap the two matting surface's together?

Where exactly is it leaking from ? From the threaded connection of where the two parts of the union come together (the large nut ) ?

It is leaking from the top of the threads where the nut fastens to.
But it would be shown there it the 2 mating surfaces are leaking.

Well, you have to remove the union, re-apply thread sealant ( pipe dope, teflon tape, etc. ) and re-tighten it. No way around it. I would not suggest you try to solder the thread to union.

There seems to be some controversy in using pipe dope.
I didn't use it here and that is probably why it is leaking.

Controversay, not from us. We use pipe dope all the time, especially if you know you already have a leak.

Yeah...

I usually put pipe dope on the threads only of a union, but if it leaks, I'm pipe doping where the union ground joint mates up to to each other and tightening all up.

Hope you're all set!

MARK