OP, did you use an old toothbrush with the citristrip? I usually cake it on, let dry, scrape/rub the top coat of paint off, cake another layer on, let dry again, then add just a small amount to get everything liquid again before brushing with a toothbrush or nylon brush. Works every time.
If that doesn’t work, there aren’t any chemicals I know of beside strong acids and bases that will remove the oxidation layer. Typical solvents like benzene, toluene (stuff that’s in carb cleaner, or at least make up the majority of it), alcohols, water, etc have no chemical interest in the oxidized aluminum so they’re general safe to use. Mineral spirits or carb cleaner are just fine and will get the job done.