Looks like oxidation from failing clearcoat to me. As clearcoat fails, it sometimes peels (or flakes off) like a sunburn which leaves the aluminum underneath exposed. The longer portions of the aluminum is exposed, the more oxidized it becomes. The clearcoat failing in patches, leaves oxidized patches. It should polish out - but as you suggest, probably not in one pass. But no trailer polishes in "one pass" - it's a multi-step process.
Shari
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