What Does
Cratering Mean?
Cratering, a coating defect, is the formation of small bowl-shaped depressions in a coating film. These depressions frequently have drops or bands of material at their centers and raised circular edges.
Causes of cratering include:
- Gel particles
- Dirt
- Fibers
- Undissolved silicone
- Overspray
- Oil contamination
- Substrate contamination
Cratering plays a very important role in the coatings industry. It has been established that cratering can lead to catastrophic fracture under the cycle load.
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Corrosionpedia Explains Cratering
Cratering, or craters, are spots where paint on the coated surface has receded, often concentrically, to leave a spot with no paint. Cratering is caused by incompatible foreign materials either in the paint or on the substrate. Fish eyes are craters that have a bit of material remaining in the center. Cratering can also occur when trapped air or solvent bubbles which have burst leave small craters as the coating dries, leaving the coating insufficient time to flow into a uniform film.
The reason for the formation of craters is the difference in surface tension between the liquid paint and the contaminant. Such defects increase as surface tension differences grow. They can be avoided by lowering the surface tension of the liquid. Pinholes are craters that develop where the liquid has not formed a homogeneous layer, thereby leaving a depression which penetrates to the substrate.
Cratering can be avoided by improving spray technique — applying a mist coat and avoiding air entrainment during mixing. Thinners should be added as recommended by the paint supplier. Cratering can be repaired by abrading and cleaning the surface, then recoating.