As a follow up to the gas-fired torches, irons and salimoniac I finally dug to the corner of the shed where the special fluxes have been stored for a number of years so here is the brand I've used extensively. Stevencity and soundguy were discussing where to buy Stainless flux, heavy percentage of hydrochloric acid.
I've used it for stainless steel repair and also for copper to stainless steel connections. One time Dad wanted to make an experimental wine keg out of a 16 gallon stainless beer keg. I had to remove the tappers fitting and solder in a 1" copper male adapter to act as the 'manual' tapper and additional fittings on the side of the barrel for filling with the grapes and condiments.
Johnson manufacturing from Prineton, Iowa. It sure makes it a much easier task over normal fluxes. At least that's been my experience. This particular bottle was purchased from a sheet metal supplier here in StLouis called Three States Supply, suppliers of galvanized sheet metal, copper flat stock, plus flat stock stainless steel of all various guages. Sheets of mild steel from around 16 ga. to 1/2" thick.....and of course, many more items and supplies too numerous to mention.