Although this is an extreme case, Thermal Spray Coatings (TSC) or metallizing can exhibit a mottled appearance due to application technique, the timing before the application of spot coats to build thickness, and the smoothness/roughness of the film. The appearance will blend somewhat over time as the metalizing oxidizes, but in this case it will not become completely uniform.
If the TSC is not sealed or painted, some discoloration and light rust stain in the porosity of the coating can form over time. If the exposure environment is mild and the porosity of the film is not excessive, this may not be a problem, but if the coating is extremely porous and the service environment involves high humidity and moisture, rust staining can become heavy and pose problems with performance.
To avoid potential problems with rust staining, the TSC should be sealed. If the objective is to maintain a gray metallized “industrial” look, this can be done by applying a clear seal coat (e.g., urethane) over the surface. The first application is thinned as allowed by VOC regulations to penetrate and seal the porous film followed by the application of an additional few mils of the clear coat that is properly thinned. In this case, the use of a clear coat would lock in the mottled appearance, preventing any chance for the patches to blend together to some degree over time.
To avoid this altogether, owners will often specify the application of a thinned epoxy coat to seal the porosity, followed by the application of color coats. If VOC restrictions prevent the epoxy from being thinned adequately to penetrate the surface, a 100% solids epoxy penetrating sealer can be used. In either case, the goal of the first coat is to seal the surface, not to build a film above it. After the color coats are applied, the structure has the appearance of any other painted surface, but with the added benefit of a heavy thickness of TSC beneath it.
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