Q. I am a manufacturing engineer at a plant that makes insect screening from aluminum alloy 5052. We apply paint to this screening by a dipping process, then use a high-speed rotating beater to beat away excess paint from between the wires. The finished product shows some bare spots on the surface, and we don’t know how to solve this problem. K.U.
A. Bare spots on your painted wire screen are probably related to poor wetting. This could be caused by either 1) improper surface preparation or 2) by bubbles of air formed on the mesh surface because of the angle at which it enters the paint in the tank.
In scenario 1, surfaces must be free from oily soils to be wetted by paints. Since the paint will not wet any contaminated areas, they will be not be coated. To solve this scenario, review your cleaning operation and make sure your chemicals are up to specifications.
In scenario 2, the surface tension of the paint could be forming air bubbles in certain areas as the screen mesh enters the paint surface in the dip tank. This would therefore not allow the paint to flow into that area, and that area would be bare.
To solve this scenario, review the dip painting operation and make sure the angle at which the screen is entering the tank precludes entrapment of air bubbles. Also, check the viscosity of your paint. Control of viscosity in dipping tanks should be closely monitored, because of solvent loss from the paint surface. I suggest you ask your paint supplier about additives to increase wetting. This may also help alleviate the problem.