Porous ceramic catalyst supports or filters to be provided with catalyst coatings via oxide washcoating processes are pre-coated with polymer barrier layers to prevent washcoat nanoparticle intrusion into the microcracked and/or microporous surfaces of the ceramics, the barrier coatings being formed of hydrocarbon polymers that are soluble or dispersible in polar media, capable of forming neutral or hydrophilic surfaces on porous ceramic supports, and completely vaporizable at moderate washcoat stabilization or catalyst activation temperatures.