Andrew R Stewart 

Gas Fluxer- I used one (actually one of two different brands) for a few decades before I retired the last one about 15 years ago. Sold it last year. Flux does nothing to change why the filler wets out but everything to do with keeping the base metal oxide free so how the filler wets will work as intended. I found that using the fluxer only made for frustrating brazing as the base metal could more easily become carbon coated (or whatever the black stuff is) or a too hot spot would burn more quickly. Paste flux only works, as most here already know, quite well. Both together seemed to extend the flux's "life" and slightly reduced the amount of blackening or burning, but only slightly less. Clean up with both is about the same as paste only, maybe slightly nicer as the bad spots might be lessened.

But to repeat it's my understanding that the flux, whichever method is used, only keeps the metal clean to accept whatever filler is being used. It's the filler/base metal and skill with the flame that determines who nicely the brazing ends up. Andy