Paint Too Thick
Q. We manufacture durable equipment used out doors. To save energy, we paint our products with air drying paints. We apply a three coat finish system, primer, intermediate coat and a topcoat. Each coat is applied at a nominal film thickness of one mil (0.001 in). The final dry film thickness of the finish system ranges from three to five mils (0.003–0.005 in). One of our customers requested a 10–12 mil (0.010–0.012 in) final dry film thickness for what he considers to be an extreme exposure application. After several days the finish is still soft and has a wavy appearance. What is causing our problem? B. A.
A. The film softness indicates it is not cured. The waviness indicates the film sagged. If you applied ten to twelve mils of paint in three coats, you probably did two things wrong. You not only applied too much paint per coat, you also allowed insufficient drying time between coats. Drying time between coats, as specified by the paint supplier, is generally based on a one mil (0.001 in) dry film thickness. If you increase the film thickness significantly, a longer time to recoat is required. The best procedure to follow when a greater than normal film thickness is required is to apply more coats at your normal film thickness per coat. In other words, if you must apply a finish system with a dry film thickness of 10–12 mils (0.010–0.12 in), apply it in one mil coats, following the supplier’s recommended ‘time to recoat’ specification.
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