Computer modelling of the structure of metal-filled compositions, allowing the efficiency of utilization of the metal filler to be estimated, was carried out. The size of the A-type 'infinite' clusters, electrically connected to the substrate, was shown to be proportional to the electric conductivity of the paint. In a numerical experiment, we determined the share of the surface covered with the conductive clusters that characterizes the substrate's protection degree. The actual protective effect was found to be also related to B-clusters contacting the substrate, but having no outcrop.