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I would just strip it down using a paint stripper and then paint again in your desired finish. I hope I understood your question correctly. If it's copper though, the patina actually forms from weathering...so that is totally different than painted aged bronze.
Rinse in warm water and gently clean with a soft brush like a toothbrush. Make a baking soda paste with lemon juice and rub onto the metal, leave for about 20 minutes. Rinse with distilled water. Repeat as needed.