In Australia a long-term project is studying the marine atmospheric corrosion performance of several metals and alloys, and a range of generic classes of coatings and finishes (metallic and organic) on steel and aluminum. Specimens have been exposed for over nine years at three marine sites, both in the open and under a specially designed glass shelter which has greatly accelerated deterioration. The sheet product test pieces include a range of features designed to simulate the types of distress introduced during manufacture and building, and at which corrosion defects initially manifest. A methodology is described for the development of quantitative performance indices for these formed specimens. The general philosophy involves using the collected data to determine appropriate weighting to different types of defects on features, or to the relative contribution of different features to overall defect indices and in turn their relative contribution to a total performance index. A rigorous mathematical procedure was followed to calculate defect indices and total performance indices for all products. Some results for the two-year specimens are discussed.