Google "braze stop off". "Stop off" is the key term. We used to mix
up several kinds for vacuum brazing applications. basically a paste
made from stuff like aluminum oxide dust and a vehicle, sometimes
alcohol. Could be water, I suppose.

Pete Stanaitis
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Cydrome Leader wrote:
What's a good way to keep solder from flowing or wetting copper or brass
parts being soldered- sort of like an anti-flux. I recall seeing something
posted here about it, but can't locate it.