I broke down the kidney grilles yesterday and gave the rings a deep clean. This time I chose the 1" mandrel for a standard drill with M205 polish. This minimized the random oscillations, since I really didn't have a good way to secure the rings, so I had to hold them while I was polishing them.
Here's a good representation of the oxidation that was on them. See the line? The white spots were just dust, but it cleaned up really easy.
Here's a side by side of the finished ring and the unfinished.
And the finished grilles...