The small bowl-shaped dents that form on the surface of the paint film and resemble tiny volcanoes are called craters. In the center of these craters, the presence of either drops or lumps of material is usually observed and the boundaries of the craters are elevated. 

Fig 1.

Fig.1: A typical crater with some material in the center and characteristic high edges 

Fig.2: A cross-sectional view of a clearcoat crater with a diameter of approximately 100 μm

Typical causes of crater can be listed as: Gel particles, contamination, fibers, filter material, undissolved silicone oil, overspray, oil droplets from air-conditioning lines or machinery, and contaminations on the paint applied surface. 

The schematic cross section of a typical crater is shown in Fig. 3

Fig 2.

Foreign particles or contamination (impurities) are the driving forces of crater formation and are usually located in the center of the crater. Silicone compounds which are incompatible in the paint, oil droplet, or solid particles contaminated with them are typical contaminants causing craters. Paint moves from the low surface tension areas where the contamination is located to the higher surface tension areas resulting in the formation of a crater. The driving force of this leveling is resisted by gravity and paint viscosity. The interaction of these forces results in the typical circular elevation around the crater. Granular substances (such as dust present in the environment) can cause craters due to absorbing low surface tension materials such as solvents, monomers, oils. However, some impurities, such as fiber, may cause cratering itself due to it’s low surface tension.

Craters can generally be prevented by improving the cleaning controls and cleaning efficiency during paint production and paint application. The use of oil trap filters in the lines of paintshops that feeds the air to the paint application guns and bells,periodic maintenance reduce the possibility of oil contamination that can occur during application by spraying. Proper ventilation and air control also reduce dust and the amount of spray dust.

In addition to eliminating external causes of cratering, paint can also be formulated to have higher crater resistance. A low surface tension paint is more resistive to crater formation than a high tension product.

Fig 3.    

Fig 4.

The wetting, leveling and covering performance of the paint increases as the surface tension decreases. As a result, to wet and cover not only the surface but also dirt / spray dust will be wetted and covered, which prevents cratering. High viscosity of the paint also slows down surface flow until film formation and delays crater formation. Although these attempts reduce crater formation, it is generally necessary to eliminate or reduce external factors (pollution, gel particles, etc.) to completely solve the crater problem.

Some paint researchers tend to name one type of crater separately as Fisheye. Fisheye defect is caused by a low surface tension liquid material present in the paint. This is usually an oil droplet, a silicon oil droplet that is not well dissolved and thus not homogenized in the paint, or a defoamer additive. Therefore, in the fisheye defect, there is no solid material in the middle of the crater; however, there is a uniformly spread liquid (which can be judged as paint as it partially mixes with other paint ingredients). It is surrounded by the typical dent followed by a circular elevation.

Fig.4: Suface image which represent Fisheye problem

Fig 5.

Fig.5: Possible defects and improvement actions for Crater problem(
The liquid can dissolve better in the paint over time, the initially fisheye defect may disappear or get smaller as the paint ages. Filtering is also sometimes helpful if an absorber filter is used. The most effective measure is to identify the root cause of the fisheye and remove the incompatible material from the formula or the environment.