A method of coating a filter substrate 4 comprising a plurality of channels comprises: introducing a pre-determined amount of liquid 5 into a containment means 2 at an upper end of the filter substrate; and coating the channels having open ends at the upper end of the filter substrate with the liquid from the containment means. The coating step may be carried out by the applying vacuum to a lower end of the filter substrate. An apparatus 1 for coating a filter substrate with a liquid comprising: a containment means for receiving a predetermined amount of liquid; a liquid dose in head arranged to dispense the predetermined amount of the liquid into the containment means over an upper end of a filter substrate; wherein the containment means is locatable at an upper end of the filter substrate and the liquid dose in head comprises a plurality of apertures for dispensing the liquid onto the upper end of the filter substrate is also disclosed.