A common paint defect is high film buildup of paint along the edges of a panel or metal part (fat edge), resulting in a picture frame appearance due to the paint build up on the panel edges. Paint build-up can be commonly observed in flow and dipping paint applications, but this phenomenon can also be seen in spray - bell applications, roller applications and even electrophoretic coatings. In dipping and flow applications, paint builds up on the edges, especially at the bottom area. In order to remove excess paint from the surface, main precautions can be listed as: extending the drip times, reduction of the paint viscosity, or using electrostatic or centrifugal forces. Sometimes, paint accumulation occurs due to sagging. If this is the case, the use of fast solvents or additives such as thixotropes can be effective in reducing the tendency to sag.
Although there is no sagging or framing observed during the application, framing may occur during the baking or drying process. This phenomenon occurs along all four sides of the panel or part being painted, so it cannot be explained by gravitational forces. It is a result of surface tension and/or surface tension gradients. Solvents usually evaporate from the edges at the fastest speed. Because, throughout the panel, evaporation takes place only at the paint-air interface parallel to the panel. When the edges are considered, the wet paint present on the edges has two air interfaces, one parallel to the panel and the other perpendicular to the panel. Since the surface tensions of organic solvents are generally lower than the surface tensions of the other ingredients of the paint, a surface tension difference occurs, which causes the paint in the middle parts to migrate towards the edges and as a result, the film thickness in this region increases. This type of "framing" can be prevented by adding silicone or acrylic-based surfactant additives that reduce the surface tension differences. (Fig.1: Tendency of paint to escape from sharp edges to minimize surface area. Insufficient coating of the edges reduces the protective and decorative function of paint.)
Surface tension tends to reduce the surface/volume ratio of the paint. Accordingly, the coating film will tend to take a shape that will minimize the surface area. On flat surfaces, this tendency causes brush marks and roller lines to vanish by leveling of the paint. On sharp edges, the paint may undergo a surface area reduction that results in thin edge coverage. This low film thickness at the edges leads to the formation of a thick paint layer looking like a strip, a little further inside the edge, resulting in a typical frame look. As a result, very thin and very thick coated areas follow each-other on the surface. Insufficient corrosion and weathering resistance are observed on the thin film thickness area whereas surface appearance quality is getting worse on the thick film thickness areas. Silicones and surfactants are effective in such cases only to the extent that they reduce the surface tension of the paint. However, the driving force to reduce the surface area of the paint still remains functional.
Thickening agents are more effective hence they provide resistance against the escape from the sharp edges due to surface tension gradients in the paint. Generally, the combination of a thickener and a surface tension reducer is the most effective way to reduce or completely eliminate the formation of framing.
In electrostatic applications, the coating of the paint on the surface is directly proportional to the conductivity of the paint. Paint can be coated even on the back surface of the panel or part during the application of the higly conductive paint. Relatively, more paint accumulates on the edges and corners as a result of the efforts of the paint particles outside the coating area as paint dust to adhere to the metal surface with electrostatic charge. Such situations can also cause framing.