Good paste Flux, old school tin/lead solder. Some 400 wet / dry 3M emory cloth.
Well, that's what doesn't work, unless I sand off the finish on the inside surface. That is a bit difficult to do well, unless I open the "u" further (it's closer to a "C", really), sand it, then bend it back to the shape I need. The pieces are small enough to be a bit hard to hold on to while sanding. Possibly a small wire brush attachment on my Dremel tool would work better than trying to sand by hand. But, I'm just puzzled / curious as to the problem. I've done a lot of copper pipe soldering in the past, but that was to the outside surface of the pipe or tube, slipping into some fitting. Never had a problem with the inside surface of a fitting, if new. Anodizing the inside surface of the tube WOULD sort of make sense to extend its lifetime. Or maybe this is oxidation, but not to the point of darkening or "greening" the copper? This looks very "even" though - not "patchy" at all.
Yup, long as drinking water will never pass through it old lead solder can be easier to work with.
Copper is a fantastic conductor of both heat and electricity.
It makes it hard sometimes to get the copper hot enough to
let the solder adhere though... a torch is usually needed to
get the temp up... and lots of flux.
Silver is even better as a conductor of heat and electricity
and if it was as plentiful as copper would be used instead
in many applications. I have seen silver heatsinks used on
certain critical hardware... I will have to stick to aluminum...lol
The treasury sent the military most of its silver in WW2 for wiring used in the Manhattan project.