CoatingsPro Magazine

MAY 2015

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Page 76 of 92

76 MAY 2015 COATINGSPROMAG.COM showed several areas with dirt and contamination, which can account for poor adhesion. Cross-Sectional Microscopy and Coating Thickness Measurements Cross-sectional microscopy and coating thickness measurements were performed on each of the following retained coating samples: Area No. 1, Area No. 2, and Area No. 4. Te samples were mounted and polished follow- ing standard ASTM metallographic preparation procedures. Te cross-sec- tional examination of each coating sample revealed inconsistent coating thicknesses. Te coating thickness measurements obtained from cross section microscopic examination are provided in Table 1. Photo 1 is a photograph of the back side of the delaminated coating sample retained from Area No. 2. Photo 4 is a micrograph showing the condition of the back side of the delaminated coating sample from Area No. 2, exhib- iting a dirty surface. Photo 5 is a cross section micrograph showing a coating sample from Area No. 2 with coating thickness measurement of only one coating layer and extensive porosity, which is an indicator of improper appli- cation and curing. Te coating application appears to exhibit an inconsistent coating thickness that does not appear to meet the manufacturer's application recom- mendations for best performance. Furthermore, the microscopy of each sample revealed extreme porosity, which an indicator of an inadequate application. In some cases, extreme porosity can lead to poor adhesion. Conclusion Based on the onsite inspection and the laborator y analysis of the coating failures at the elementar y school, the failures appear to be adhesive related. In this case, the failures were primar- ily between the preexisting coatings and the newly applied primers. T he laborator y analysis determined that the failures appeared to be caused by inadequate and inconsistent coating application. T he application instruc- tions for the primers used clearly stated that the proper surface prepa- ration should be sound, dr y, clean, and free of oil or grease, rust, mildew, loose and or f laking paint, or other foreign substances. Painting over preexisting coated surfaces is not recommended. In some cases, sanding glossy or hard surfaces is required. T he results of the analysis determined that the application instructions were not properly followed, resulting in the adhesive failures obser ved at the elementar y school. Additionally, while we were doing our observations, Area No. 3 was just about to be painted. We checked the surface with a black felt cloth to reveal an improperly cleaned surface. Tis observation, along with the other observations, is a strong indication that the newly applied coating may result in adhesive failure, too, if not properly prepared. Photo 3. This photo shows Area No. 3, which was in the process of being painted over. It shows what appeared to be a glossy and dirty surface with no sign of abrasive surface preparation. Table 1: Sample Area Individual Thickness Measurements (mils) Average (mils) Area No. 1 Primer: 4.47, 3.94, 3.83 Topcoat: 5.10, 5.35, 4.39 Primer: 4.08 Topcoat: 4.94 Area No. 2 Primer: 5.00, 5.10, 5.15 Topcoat: N/A Primer: 5.08 Topcoat: N/A Area No. 4 Primer: 11.56, 11.87, 13.00 Topcoat: 2.22, 2.17, 1.92 Primer: 12.14 Topcoat: 2.10 Never Again

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