I soldered some 3/4" pipe and didn't like the looks of some of the joints;
there were a few spots that didn't show solder and I don't want any leaks..
So I took them apart. The insides were completely tinned on both parts, so
the joints were probably okay. (Is that a fair evaluation?) But after I
cleaned them up they wouldn't go back together again because of the
thickness of the solder. I heated them up and worked them back together,
and added a little more solder. Now the joint looks fine.

Is it? I didn't use any flux the second time because it just would have
fried off, but since the joint was already tinned from the first time, that
shouldn't matter should it? The flux is just to get the solder to flow over
the copper, so it shouldn't be necessary on tinned copper. Right?