This relates to the manufacture of wet process hardboard. The invention involves supplying additional heat curable resin to the mid-thickness regions of the panel by injecting additional resin into the moving mat as it is being formed on a forming surface. The region of injection of the resin is selected so that the bottom of the partially formed mat is sufficiently consolidated to resist the passage of resin therethrough while at the same time the top of the mat is still sufficiently liquid so that disturbances of the fibers of the partially dewatered mat caused by the injection of the resin have an opportunity to mend. The resin may be injected into the longitudinal marginal portions of the mat in continuous fashion such that, after the edges of the final board product have been trimmed, the resin treated areas are exposed to view. The additional resin reinforces the edge portions and assists in providing uniform caliper of the board across its width. The resin may also be injected intermittently so that the resin-treated areas coincide with the corner portions of the mat after it has been cut to length thus reinforcing and assisting in providing uniform caliper as between the central portions of the board and the corner portions. In a further variation, the resin may be injected into the moving mat along lines spacedapart to coincide with the lines of cut of saws used to produce narrow strips of panelling which are later fabricated into lap siding. Each narrow board product ultimately produced has additional resin along its marginal edges and thus has additional strength and resistance to moisture penetration thus making it particularly useful for lap siding and the like.
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