Metal Finishing Guide Book

2012-2013

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10. Watching metallic flakes separate. The owner of a paint shop complained that no matter how his painters applied a particular pearlescent metallic paint, the final finish always appeared mottled. One of the more curious and possibly exciting observations is to watch metallic paint to dry. In this case, a few drops of a pearlescent metallic paint were applied to a glass microscope slide. Viewing these drops through the microscope, you can immediately see the metallic particles flow along the surface of the paint toward discontinuities, such as small air bubbles, tiny dust particles, etc. The flow of pigments looked much like large quantities of debris flowing down a fastmoving river. In this case, over a period of a few minutes the metallic pigments started to form cells (as shown by the arrow in Figure 13), which kept growing over time. By the time the paint had dried, the entire surface was covered with large cells, resembling B��nard Cells. When looking at the painted panel from a distance of approximately 2 to 3 feet, the surface had a mottled appearance and did not meet the client���s requirements. Because separation occurred so quickly after the paint was applied, this investigation highlighted the importance of agitating the paint aggressively during paint application. Therefore, the use of a gravity-fed or cup spray gun would probably not be able to prevent such separation. Even the common air-driven agitators, used in most pressure pots, likely turn too slowly to prevent separation. On the other hand, a squirrel cage mixer may suffice. If the painter stops triggering for more than a few seconds while moving from one part to another, the pigments might already start to separate in the fluid hose. To prevent separation after the paint has been deposited, it would behoove the painter to use a higher-than-usual atomizing air pressure and achieve a dryer rather than wetter finish. The goal should be to flash off the solvents quickly, thereby preventing the pigments from having too much mobility. When one encounters a problem such as this one, the paint manufacturer and painter should work together to ensure that when the painter follows the best application practices, the pigments do not have time to separate. The manufacturer might be able to reformulate his product using faster drying solvents, or perhaps by selecting pigments more compatible. The painter has several tools at his disposal to apply a dryer finish, thus helping to prevent pigment separation. CONCLUSION This paper used illustrated examples to demonstrate the usefulness of tabletop optical microscopes. Such microscopes enable a paint consultant to better understand why a paint failed, if specifications were followed, and often explain any unacceptable appearance. 588

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