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45 NACE INTERNATIONAL: VOL. 56, NO. 4 MATERIALS PERFORMANCE APRIL 2017 Continued on page 46 coating types (i.e., acrylic, alkyd, epoxy, epoxy phenolic, epoxy zinc, organic zinc, in- organic zinc, metalizing, moisture curing polyurethane, and miscellaneous coatings); types of surface preparation—hand or power tool cleaning or abrasive blast clean- ing; number of coats; and minimum dry film thickness (DFT) of the coating. Coating types are grouped into categories for either atmospheric exposure or immersion ( water) service. Coating life-expectancy informa- tion corresponds to the corrosivity of the service environments. e harshness of at- mospheric service is classified as C2 through C5 for mild, moderate, severe (heavy industrial), and seacoast heavy in- dustrial, as defined in ISO 12944-2, "Classifi- cation of Environments." e service envi- ronments corresponding to immersion service include potable, fresh, and salt water immersion. Generally, the authors say, most users follow a maintenance painting sequence of spot touch-up and repair, then mainte- nance repaint (spot prime and full coat), and finally full repaint (total coating re- moval and replacement). ey estimate the number of years for a practical mainte- nance sequence as follows: spot touch-up and repair at the practical life (P) of the coating system, maintenance repainting at the coating system's practical life plus 33% (P x 1.33), and full repaint at the coating's practical life plus 50% (P x 1.5). Helsel and Lanterman emphasize that distribution of coating breakdown must also be taken into account when judging the costs and feasibility of maintenance painting. "For example, 5% breakdown that occurs in well-defined areas can be practi- cally repaired through localized touch-up, whereas 5% breakdown uniformly scattered across 100% of the surface may be beyond practical spot repair," they say. Also, the authors point out that the practical maintenance sequence may not always represent the most economical ap- proach to maintenance painting. e physi- cal characteristics of the existing coating and the amount of corrosion present are the determining factors, and it may be pos- sible to perform several touch-up and main- tenance repainting cycles and push the time until full repainting is required. ey note that the decision to conduct a mainte- nance repaint vs. a full repaint should be based on results of a coating investigation that assesses coating thickness, adhesion, substrate condition, and the extent and dis- tribution of corrosion.