A method and apparatus for applying a uniform membrane coating to a substrate, such as a honeycomb structure, having a plurality of through-channels, wherein the through-channels have an average diameter of less than or equal to 3 mm. The method includes providing a liquid precursor comprising membrane-forming materials to the substrate and applying a pressure differential across the substrate. The pressure differential causes the liquid precursor to travel uniformly through the through-channels, depositing the membrane-forming materials on the walls of the through-channels and forming the membrane on the walls of the through-channels. The apparatus includes a chamber capable of holding the substrate and of maintaining a pressure differential across the plurality of through-channels.