Silking is a kind of defect that occurs in the appearance of thin, parallel lines that give a silky effect to the surface of the coating film. They usually appear on the coated film as a special type of a floating that performed by dipping or flow coating applications. While the coating is dripping off from the surface, if there are any Bénard cells, they will elongate like a thread. The phase separation depending on pigment aggregation, resin incompatibility, or other insoluble or incompatible components can also result in similar appearances in the dipping and flowing applications of the film. In the coatings applied with brush, the segregation of coarse-grained filler pigment particles with the direction of the brush movement cause to a silky appearance. A microscope can be useful with understanding of these reasons.

Fig 1.
The silky effect formed by the Bernard cells can be prevented by increasing the viscosity, adding surface additive, and choosing pigments that are close to each other about their densities, primary particle sizes and agglomeration degrees to reduce the pigment mobility in the wet coating film. Silk appearance due to incompatibility or lack of dissolution can also be resolved by changing the solvent or removing the incompatible resin or other components from the formula. The silky effect that occurs in brush or roller applications can be avoided by removing the coarse-grained fillers from the formula.

The “discoloration by rubbing” defect refers to the problem of surface discoloration when force is applied to the painted surface, for example when the vehicle surface is polished. This problem may be caused by agglomeration, floating or flooding of pigments in wet paint. In order to understand the existence of this problem, the rubbing test, called "rub-out", is performed. This test is performed by rubbing a finger on the wet paint film applied to the glass surface. Depending on the drying mechanism of the binder in the paint, the rubbing process is done after the film is left to dry in normal conditions for 30 seconds to 5 minutes. It is important that the paint is not completely dry while performing the test. While testing, it should be ensured that a force that does not create gaps in the paint film, and that the paint is not pushed to the sides of glass during rubbing. If the pigments are not sufficiently stabilized, the rubbing forces disperse the agglomerates, increasing the surface area of the pigments, so the color intensity of the pigment increases and a visible color change occurs on the surface.

At the end of the rubbing process, if there is whitening in the area where the force is applied with the finger, this indicates that the titanium dioxide pigment agglomerates. If there is a color shift towards the color saturated side, the color pigment becomes agglomerated. In Figure 1, the color change in the rubbing areas indicates that the titanium dioxide pigments agglomerate. In the Figure 2, the color change in the rubbing areas indicates that the green pigments agglomerate. If it is desired to make a quantitative control other than the qualitative control by visual assessment, it can be accepted that the cases where the color difference between the force applied and the unapplied side is at the level of dE≤0.3 wil not cause paint defect.

If the cause of the problem determined by rubbing is floating or flooding, the problem can be solved by reducing the amount of pigmentation at a level that will not affect the covering of the paint, or by reducing the mobility of the wet paint with rheology additives. If the cause of the problem is pigment agglomeration, the solution is to eliminate the cause of this agglomeration. To solve the agglomeration problem, wetting and dispersing additives related are usually required. Sometimes the cause of agglomeration is the pigment or binder shock that occurs during the lte down process. Whatever the reason, the root of the problem should be determined and the color difference caused by rubbing should be eliminated.