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I have used Brasso before to get the stains off the brass but that was a long time ago. I do not like the liquid adders because I have had them to form clumps that became stuck inside the cases. RCBS makes a dry powder polish that will really make the brass shine.
I think after tumbling a few hours in walnut you are down to only a cosmetic issue. As long as they are clean enough to protect the dies I load them.