Removing Shadowing On Aluminum From Old Signage

Have 20ft wide area of shadowing left from an old sign that was replaced. Its on painted corregated metal. I know that the shadowing from the old letters is where there is no oxidation and the sun never hit it. But I want to see if what may work the best to remove the shadowing. So far I have had the best luck with some paint deglosser. Wanted to get some other ideas.
Thanks

Besides oxidation you can also have color fade which goes deeper in the paint.

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