Have you ever applied a three coat system (inorganic zinc/epoxy/urethane) and seen pinholes in the epoxy? Applying coatings over inorganic zinc primers can easily lead to the development of pinholes if precautions are not taken. Inorganic zincs are mostly zinc in the dried film, which provide plenty of room for air to hide between the zinc particles. During application of the intermediate coat, this air attempts to escape as the coating works its way into the zinc primer. As the air leaves the intermediate coat, the bubbles burst, causing pinholes or craters that may remain in the dried coating if it fails to coalesce prior to drying.
How can pinholes be prevented? The application of a mist coat (dust coat) is the most common technique. This is a light coat which allows the coating to penetrate into the zinc and displace the air, but it is applied thin enough to allow the air to escape the mist coat easily. A full coat is then applied after a short waiting period while the air escapes. Many applicators also prefer to thin the coating used for the mist coat more than they thin the full coat. The coating used for the mist coat must be thin enough to penetrate into the voids of the zinc coating.
One coating that has been specifically designed for this purpose is International’s Intergard 269. In many specifications, Intergard 269 is often added to as the second coat in a four coat system. This coating has a low viscosity with 47% solids by volume. When used over inorganic zincs, it should be thinned 15 – 25% for good penetration into the zinc. Intergard 269 is very effective at eliminating the occurrence of pinholes.
When you are trying to prevent pinholes over inorganic zincs, low viscosity and a light application of the subsequent coat will be helpful.