Metal Finishing Guide Book


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Figure 13. Metallic coating shows separation into cells structures as the paint starts to dry and cure. mately 15X magnification. To highlight the cavities, the paint consultant chose to illuminate the sample paint chip at a shallow oblique angle of approximately 10 to 15 degrees. Using oblique lighting, the top of the coating clearly showed pinholes filled in, but nevertheless, left small craters on the surface (Figure 10). 8. Paint blistering Figure 11 illustrates what one might typically see under a large paint blister. In this micrograph, a large blister formed between the green top coat and the epoxy under coat. Severe solvent entrapment in the gray epoxy probably caused solvents to attempt to escape through the top coat. If the top coat was set up and cured, it would have been impermeable to some degree to the solvents lead to blisters. Elsewhere on the painted equipment, the top coat had already ruptured and peeled. 9. How many coats? Paint was peeling from exterior wood siding. Upon closer examination, it was clear the coating was pulling the wood apart but that the paint itself was intact. The upper surface of the wood was aged so it could not withstand stresses applied by paint. How could one or two coats of acrylic latex paint apply such stress on wood? When a sample of the painted siding was examined under a stereomicroscope, it was evident the building had been painted five times (Figure 12). Every time a new coat of paint was applied, additional internal stresses built up. In fact, the total DFT of the system was 19.5 mil. It is understandable how so many layers could generate sufficient stress that the aged fibers of wood would lose cohesion and tear apart. 532

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