I enjoy soft soldering like [similar] brass/bronze/copper....or any
RED metal and use Bakers zinc chloride based liquid flux
Gas flame heating of like
RED materials all have similar characteristics in absorbing heat, whereas any of the 300 Grade austenitic series stainless metals have opposite heat reflective tendencies ....all of which do not engender for maintaining the fluid or unburnt state of the flux
From this, I find considering the soft soldering of composite materials such as stainless and brass as a process best not attempted if at all possible
Another consideration would be the cost of the phosphoric acid at 32.50 Pounds UK for as a bottle a little expensive
....and would I ever use this flux again?
...........
Derek