A method for rigidifying a fiber-based paper substrate for use in the exhaust system of a combustion device. In the method, a green ceramic fiber-based paper substrate is impregnated with an impregnating dispersion. The impregnated substrate is then dried, calcined and fired to form a rigidified substrate that is suitable for use in the exhaust system of a combustion device. This rigidification process is performed at least once and, preferably, two or more times. The green paper substrate comprises two or more sheets of green ceramic fiber-based paper, with at least one creased sheet and another sheet being a laminated together to form a plurality of tubular channels. The rigidified substrate comprises refractory ceramic fibers in the form of a ceramic fiber-based paper and agglomerates of ceramic particles. The ceramic particle agglomerates are bonded to and disposed so as to thereby bond together the refractory ceramic fibers at spaced locations along and at intersections of the refractory ceramic fibers so that the refractory ceramic fibers retain much of their original flexibility while in the paper.