I never did like that Batterns flux too much! I now exclusively
use that thick white "handy flux" which seems to work really
well if you heat it up slowly enough so the solder doesn't go
popping off everywhere.
When I made the comment you quoted I also said that the borax
was concentrating in crystals in the bottom of the bottle,
reducing the effectivness of the flux I was drawing off the top.
Batterns is still my choice for the bulk of my work. Now and then
I'll use a tan, grainy, water based paste flux a friend once gave
me a bit of. I think it's made for brazing stainless steel, I'm
not sure. Works well when I've got some white gold giving me a