Craters and pinholes are caused by liquid or solid contamination present on the substrate or in the paint or the surroundings before the drying process starts. 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.