Polymer coatings containing aligned mica flakes can reduce solute permeability by more than an order of magnitude. This permeability reduction can be used to inhibit corrosion of aluminum coupons coated with mica-filled epoxy. The extent of permeability reduction, which depends on the flakes' volume fraction, shape, and alignment, varies with volume fraction squared, as predicted by some published theories. The flake alignment can be achieved either by solvent evaporation or shear, but is independent of the shear rate used to orient the flakes. This shear rate independence is consistent with fluid mechanics calculations, though these same calculations predict better flake alignment than is found by small-angle X-ray diffraction.