Treatment of particulate filters

A method of applying a dry powder to a porous substrate, the porous substrate having an inlet face at an inlet end and an outlet face at an outlet end with the inlet face and the outlet face being separated by a porous structure, the method comprising the steps of:

• a) locating the porous substrate in a holder such that the inlet face is in communication with an inlet chamber and the outlet face is in communication with a vacuum generator;
• b) establishing a primary gas flow through the porous substrate from the inlet face to the outlet face by using the vacuum generator to apply a pressure reduction to the outlet face of the porous substrate;
• c) spraying the dry powder into or within the inlet chamber such that dry powder is entrained in the primary gas flow and passes through the inlet face of the porous substrate to contact the porous structure;
• d) during the spraying of the dry powder directing a secondary gas flow onto and/or across the inlet face of the porous substrate;
• and
• e) using a pressure and/or a flow rate of the secondary gas flow to control an axial distribution of the dry powder that is deposited in the porous structure of the porous substrate.