I SWEAR by Cuprinol. It is the best flux I have ever used. I purchase
it in bulk from Rio. I use it in a spray bottle and spray my metal,
heat my metal, spray, heat again, spray, and continue until I have a
good complete coat. It usually takes 2 or 3 heat/sprays for an even
coating. Then I solder. It really does prevent firescale and helps
the solder flow smoothly and easily. I use it on my copper work as
well as my sterling silver work.
I can not recommended it enough!!!