surfaces. A two part question. How do you clean up old rusty metal
for brazing. Is there a certain brand of flux that's worked better
for you than the coated rods?

Bead blasting works pretty well as does grinding out the cracks and drilling
the ends. Chemical methods may induce hydrogen embrittlement, not so good
for things that can kill you when they break.
I use a GasFluxer at work. Not cheap but it does an excellent job and I am
using their low fuming bronze. Works for silver soldering as well. If you
braze commercially or a lot, this is good purchase.
http://www.gasflux.com/
Some guys TIG braze, but the non-low fuming rod is hard to around here

I hope the repair you did was for a display rifle.
--
Stupendous Man,
Defender of Freedom, Advocate of Liberty.