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I had some alluminum wheel cleaner in the garage. It loosened up some stuff along with some scrubbing. But did make it's way to be bead blasted afterall.
I would try to clean as much of the built up dirt off with a pressure washer and then go to town with a wire brush. Once all the nasty is off, I would try to clean it with a good polishing compound and a whole bunch of elbow grease. Or you could just give it a good cleaning and paint it.