A process for manufacturing an elongated panel formed of synthetic materials including a polymer resin matrix, a filler and a fiber reinforcement. The process includes depositing the curable composition on a conveyor belt, distributing and leveling the curable composition prior to feeding the composition into a compression apparatus in which the curable composition is partially cured as it is compressed to form the panel. The machine for manufacturing the composite panel includes a mixer for the polymer system, filler and fiber reinforcement, a distribution apparatus that distributes the curable composition prior to feeding the composition into a compacting apparatus. The process and machine may be used to form a composite siding board that includes a grain pattern on at least one side.