Stained Hammered Aluminum - how to safely remove stains?

I have two pieced of hammered aluminum which are stained. I can polish them but nothing I've tried as far as cleaners removes the stains. I know I can't use a stiff brush, and have heard that mag rim polishes works, but is it truly safe? I guess what I'm asking is what is the "safe" way to remove the stains?

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