The present work analyses silicon oxide coated polyester films, with attention paid to the effect of substrate process additives on the coating defect population, and on the coated film mechanical properties. The defect population is characterized by means of reactive ion etching, and the mechanical properties are determined from the analysis of uniaxial fragmentation tests. The coating crack onset strain is decreased by the presence of substrate additives, due to a broadening of the Weibull defect distribution. Moreover, finite element analysis of stress concentrations resulting from additive-induced roughness shows that local stress concentrations are negligible. It is the critical defects, such as microcracks that are the most detrimental to the coating effective cohesive properties.