A method is provided for predicting the durability of a composite structure, such as a composite polymeric coating. The method includes obtaining a sample of a first (reference) composite structure and analyzing at least one layer of the structure for indications of degradation. The sample can be obtained by low angle microtomy. Degradation values and a degradation slope of the first composite structure can be defined. One or more degradation values for a second composite structure can be obtained and the durability of the second composite structure can be predicted by comparing the degradation slope of the second composite structure to the degradation slope of the first composite structure.