A method for treating a microcracked, ceramic substrate is provided that obviates the need for filling the microcracks of the substrate via a passivation step prior to sintering a layer of washcoat to the surfaces of the substrate that may either contain a catalyst, or provide a support layer for a catalyst subsequently applied. In the first step of the method, a slurry of particles of a washcoating composition is applied over the surface of the substrate such that particles of the composition are lodged into microcracks of the substrate. Next, the substrate is heat treated to between about 700° C. and 1300° C. The method is particularly applicable to ceramic honeycomb structures used as catalytic converters in automotive exhaust systems.