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The worst corrosion will happen with cast metals, they are more porous. Billet is less porous and more uniform, so polished billet will stay pretty a long time. Anyone who has tried to polish rough cast wheels knows it is all but impossible. Smooth cast is polishable, but won't usually hold it's luster very long.

Really unless it's in a corrosive environment, aluminum is a 'forever' metal, like better grades of stainless steel is.