A catalyst composition is applied to an interior of a hollow substrate. The composition is coated on the substrate by immersing the substrate into a vessel containing a bath of coating slurry. A vacuum is then applied to the partially immersed substrate. The intensity of the vacuum and its application time is sufficient to draw the coating slurry upwardly from the bath into each of a plurality of channels located in the interior of the hollow substrate. After removing the substrate from the bath it is rotated 180null. A blast of pressurized air is applied at an intensity and for a time sufficient to distribute the coating slurry within the channels of the substrate to form a uniform coating profile therein.