Using burnishing compound in a rotary tumbler, works on brass but not on steel

[GregG]

I tried to use a burnishing compound (https://a.co/d/csq4QZU) on some scrap parts that I had laying around that were either brass or steel.  I have a rotary tumbler that is typically used for rocks, and some 0.5 mm stainless steel pins as tumbling media.  I used 50 mL of water and maybe ⅛ cap of the burnishing compound.

After letting them run for 2 hours, the brass parts were immaculate; bright and shiny like they just came out of the factory.  The steel parts, however, were dull, and had a gray film on them.  A barrel arbor I threw in there was damn near black.  I would think that a burnishing compound, with the mild abrasiveness of the steel media, would stop any surface film from forming.  The product is supposed to be safe on steel, so what might be happening to the steel parts?