I've heard of "White Out" correction fluid being used for that.
Haven't tried it though.

On Mar 13, 7:00*pm, Winston wrote:
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.

I haven't tried this yet, but I hear tell that after
cleaning the heck out of pipe and fitting, one fluxes
both and assembles them, just like always.

Here's the kicker. **With a pure acetylene flame* you
coat the exterior of the joint with carbon black,
turn the oxygen back on and heat as per usual.

Apply solder to the seam and the carbon black will
prevent wetting to undesirable areas.

Clean up and the joint will have that pro look.

--Winston