A method of coating a substrate with a liquid comprising a catalyst component, the method comprising: (a) holding the substrate vertically, thereby defining upper and lower ends thereof, wherein the vertical substrate comprises a plurality of channels that are open at said upper and lower ends; (b) continuously introducing the liquid into the substrate by pushing or injecting the liquid through the open ends of the channels at the lower end of the substrate with a piston; and (c) after the lower end of the substrate has been partly filled with the liquid in step (b), applying a vacuum to the open ends of the channels at the upper end of the substrate while continuing to introduce the liquid into the substrate.