I am test-restoring some brake calipers which I presume are steel and were zinc-plated some 30 years ago. see below for some details. what this question boils down to is how to know, after cleaning the parts, if the original plating is showing through or if the plating has been worn off by either scrubbing or corrosion. I am currently testing some moisture exposure to see if rust shows up. The dust that comes off is usually not helpful, being black crud or rust. I would like to avoid having to test it by boring a hole in there and look at the layers (or absence thereof). I would like to either clean them off and use them as-is, or proceed to a plating service or set up some zinc plating electrochemistry. I would like to avoid painting them if possible. update after some interesting comments : yes I know the bore/piston surface should not be plated, is the most critical area, but that is another question, as is the rebuilding or inspection of the bore and surfaces. again, this is about the - wait for it - original plating, and possibly by extension electrochemistry or painting - but even that is for another thread I think.
I scrubbed with brake cleaner, degreaser, goof-off, then started with the brass brushes. it managed to get the caliper very shiny in one spot compared to the insoluble black coating elsewhere. there is some rust, not bad though actually. the coarse brass brush on a drill worked best for bulk cleaning. the small rotary tool brass brush was OK for small areas but took far to long to be practical.