I'm surprised to see no mention of using a "unitized" abrasive de-burring wheel.

It's basically like highly-compressed ScotchBrite material (some are even made by 3M), which makes it like a very fine grinding wheel that has a somewhat flexible surface.

So choosing a medium grit grade does produce a polished finish in very short order on surfaces with all kinds of defects and/or residual platings.
The aggressiveness of the cut is determined by wheel diameter, wheel speed (mine runs at 1675rpm), grade of the abrasive, contact pressure and finally what type of metal.

It's hard enough that it won't round corners if the part surfaces are held flat to the wheel surface.

Again, at grinding-wheel speeds, these wheels work really fast. Used primarily for de-burring and for removing sharp corners on parts at certain stages of manufacturing, even post-machning and post-casting.

The only thing is that it does usually leave a bare metal surface, so better for drier climates and followed by a wipe of light oil or wax.

I've restored many a hex-bolt head with mine. Also useful for adjusting press-fit OD's and other very light machining purposes like massaging the tips and corners of cog teeth for enhanced shifting on troublesome drivetrains. I've even used this to "adjust" for an indexing mis-match, encouraging faster shifting in one direction or the other (typically to or from a larger or smaller cog), and also to make middle and big chainrings shift with more authority in either direction.

The level of surface finish is impressive coinsidering how fast that wheel does it's work, mangled quill stem hex heads bring 'em on!

Shown at the far end of my then-new bench here, attached to one end of a standard motor. Again these wheels are available in a very wide range of abrasive grits, so can produce high polish on any metal and do it fast.