Q. I am the manager of manufacturing for a steel fabricating company. One of my duties is our paint shop. We make small- to medium-sized steel storage tanks. These tanks are painted inside and outside before they are shipped to customers. Then the tanks are filled with oil and tested before shipping. Some of the tank interior floors receive a paint touch-up after testing and before shipping because people walk on the inside during the inspection. Recently, I got a customer complaint of soft uncured paint on the tank floor when it was opened for inspection prior to filling. I can’t understand how this happened because this particular tank was shipped three months ago, and the paint we use is a fast drying enamel. How can we correct this problem in the field? What can we do to keep it from happening again?H.J.
A. Part of the paint drying process is solvent evaporation. The rate of evaporation can be retarded by solvent-laden or highly humid ambient air. To successfully paint interior surfaces of closed tanks, the residual and evaporating solvent must be removed from the air inside by circulating fresh air through the tank. Under normal conditions, spot touch-up using a fast drying paint should have dried in three months. Another possible cause of uncured paint is surface contamination. Perhaps all the oil was not cleaned from the surfaces that were touched-up. This could cause the paint defect.
To correct the problem in the field, I recommend you first try to cure the soft paint by directing a blast of hot air at the uncured touch-up paint. Using a hair dryer or heat gun may work. If that doesn’t work, remove the soft paint using a safe solvent and then repaint the area.
To prevent this happening in the future, either clean all the oil and other contaminants from damaged floor surfaces before paint touch-up or place pads on the floor before walking on it. If touch-up is required, provide for air circulation inside the tank to remove solvent vapor.