fizzy,unless you want to take it off with emery cloth, or file it, you can use either the electrolytic rust removal system, or soak it for a day or so in citric acid, or similar. Although I find that brazing with Oxy A, or propane, with a neutral flame you don't need much of a clean up at all. Ian S C

Angle grinder with wire brush- must wear goggles. Acid flux, Bakers , solder will stick to all the cleaned bits no problem.

Sorry, I read SS as soft solder. For some reason I assumed he was soldering the sheets together prior to machining.

Try dilute Hydrochldoric acid (Brick cleaner) to clean steel sheet – wash off with clean water and coat joint area with Borax paste – clamp in position and heat to just under cherry red and apply Silver Solder to edges of joints – solder will flow towards flame/heat position (Capillary action) – let cool normally – DO NOT COOL IN WATER – causes brittal joint