Try wiping it down with some Ammonia and leave it on. You might try a small area first. wrap with towel so it doesn't dry right away.

When I did archatectural sheetmetal and we worked on existing copper roofing we used urine diluted with a little water. I know it sounds gross and crazy but it did patina quite cuickly. IM sure the ammonia would probably work better and less disgusting than the aforementioned method. LOL

Dave