Try 0000 steel wool and some chrome polish. Looks like some kind of build up. Clay bar is for clear coat, never thought of trying it on chrome but would put image it would do anything.
I agree with this, and I use them in tandem with each other. After years of neglect and sitting around untended, I used chrome polish and steel wool to clean these bumpers. This is the first attempt. Another attempt is forthcoming this winter.
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