The one guy I know here that does these repairs commercially uses a
laser, and swears by it.
I’ve repaired my own and my partner’s frames a number of times.
Unless they are precious metal (unlikely these days), I use Tix
solder and flux; or on steel frames, black flux and either extra-easy
silver solder or Silvalloy brazing wire from the welding store. The
Silvalloy works particularly well, and the 40% version flows low
enough to avoid unsoldering other joints (usually). It also helps to
clamp a big heat sink right on the nearest sound joint, and get in
and out fast with a very small, very hot flame. A small alligator
clip or pair of cross-locking tweezers makes a decent heat sink.
Or you could try the old trick of burying everything but the joint
you want to solder in fine wet sand.
Of course, more and more frames these days are titanium, and you
can’t solder those.
Regards,
Bob Edwards
Chromis Designs
San Francisco