Porous ceramic catalyst supports or filters to be provided with catalyst coatings via oxide washcoating processes are pre-coated with cross-linked polymer barrier layers to prevent washcoat nanoparticle intrusion into the microcracked and/or microporous surfaces of the ceramics, the barrier coatings being formed by thermally cross-linking hydrocarbon polymers that are vaporizable at moderate washcoat stabilization or catalyst activation temperatures and being preferentially.