When dealing with especially tough soldering jobs, use N-3 Flux. This flux was specifically formulated for aggressive fluxing action when soldering chrome and stainless steel.
Where to buy
Fast fluxing action for extra tough soldering jobs
More aggressive than typical stainless steel fluxes
DO NOT USE ON POTABLE WATER TUBING
Made in U.S.A.
Use with all types of soft solders
Fed. Spec. Commercial Item Description A-A-51145C, Type 11B
Heat barrier spray
Stainless steel flux liquid
Liquid soldering flux
Soldering flux paste