So as to reinforce a panel made of cellulosic fiberboard, bands of a reinforcing material, which is cementitious, are applied via spraying, as the panel is being conveyed, so as to cover a minor portion of the overall area of a given one of the expansive surfaces, and the panel is devoid of the reinforcing material except at the bands. The bands include two diagonal bands, each of which extends between diagonally opposite corners of the given one of the expansive surfaces, two longitudinal bands, each of which extends along a respective one of the longitudinal edges of the given one of the expansive surfaces, and two transverse bands, each of which extends along a respective one of the transverse edges of the given one of the expansive surfaces.