Delamination of polymer-coated steels can easily be evaluated by the break-point frequency method previously presented by the authors. The method has been extended for evaluating the delamination of the steels having defects such as pores and fissures. The results show that the extended break-point frequency method permits measuring both the delamination area and cross-sectional areas of defects of the polymer coating. The extended break-point frequency method was successfully applied to the assessment of an epoxy-coated galvanized steel after undergoing a cyclic corrosion test for three months. The method indicated that the delamination area increased with increasing the total cross-sectional area of the defects in the coating.