I save bits of rosin purchased for use on my double
bass bow and
dissolve them in denatured alcohol. The liquid flux works just fine
when I need a bit extra (e.g. when tinning bare copper stranded
wire).
Waste not, want not. :)
I suspect that any electronics-savvy gymnast might do the same with
the powdered rosin used to improve one's grip, but to tell the truth,
I've never known an electronics-savvy gymnast.
Well, I'm certainly not one. I am very much a 'high mass hobbyist', in
both the snese that I collect high-mass machines and tbat I myself am
high-mass. Athletic I am certainly not.
But presumably anybody can buy the rosin intended for either of these uses
and instrad use it as a soldering flux.
-tony