What is a blast hole?
Bubbles that form near the surface of the paint film are called blistering. When the surface is broken but not leveled, it is a blast hole. Very small blast holes are sometimes called pinholes. Popping occurs when the viscosity of the wet film surface increases to a high level while volatile species remain underlying. The surface of the wet film evaporates quickly, making the viscosity of the surface layer higher than that of the bottom layer containing more solvent. When the temperature rises, the bubbles formed by the solvent escaping from the bottom layer are not easy to penetrate into the higher viscosity surface layer. When the temperature rises again, the bubbles expand and break through the surface to form popholes. At this time, the viscosity of the wet film is too high to flow and close the blast hole.
Pop holes can also be caused by air being trapped in a wet film, for example when brushing or rolling water-based paints, it is easy to trap air bubbles in the paint film.
What is fisheye?
Shrinkage cavities are sometimes called fisheyes. Shrinkage cavities may occur during application and painting . Coatings with high surface tension tend to shrink more than those with low surface tension. The coated surface may not be completely wetted by the coating, and this defect is often caused by grease, dust, moisture, silicone oil or other impurities. When a paint film tends to develop craters, tiny impurities can aggravate craters.
The oil and moisture on the surface of the steel plate may be caused by the unclean air during sandblasting, so an oil-water separator needs to be installed on the compressed air machine of the sandblasting equipment.
Craters usually occur before the paint dries, so it is common practice to remove craters on a wet film and repaint. If it is a cured coating, it should be roughened and smoothed after curing to smooth the shrinkage area for repair.
