I've noticed this on several old lenses I have. There is a thick, black, porous-looking but hard "deposit" on a large fraction of black-lacquered (I assume not enamel) lens barrels. It looks like adhesive residue but doesn't soften with or remove with typical solvents like kerosene.
What to I have here, and how can I remove it to show the lovely shiny black finish which I assume still lurks underneath?
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