I am in charge of painting some outdoor galvanized steel structures at our plant. These galvanized steel structures are fairly old (20 years old). They have never been painted with anything and are very dull and bare. Plant management wants to brighten things up around the plant. My question is, can I use a commercial paint in spray cans successfully on this steel without any corrosion resistant primer undercoating before I paint? Should I clean the steel with MEK (methyl ethyl ketone) before I begin? M.D.
A. One of the problems associated with painting galvanized steel is the application of an oily or waxy treatment at the mill to prevent the zinc from white rusting (formation of zinc oxide on the surface). Another problem is the reaction of the zinc with certain types of paint resins.
To passivate the surface, zinc phosphates and chromates are applied to it before painting. Old galvanized steel that has been exposed outdoors for six months or longer will be free from factory applied oils, processing compounds and pretreatments. Furthermore, the surface will be passivated by weathering and can be painted with conventional paints after the surface is cleaned. I don’t recommend the use of MEK for solvent cleaning because of its low flash point. Instead, you can use mineral spirits or another solvent that has a higher flash point.
If there is any rust present, it must be removed before priming using the same process as for rusty steel. If the rust removal process involves the use of wire brushes or steel wool, fresh zinc may be exposed. In that case, a primer especially formulated for use on zinc surfaces must be used. There are primers formulated for use on galvanized steel, such as gutters and downspouts, available in spray cans. Conventional paints can be applied over weathered or primed galvanized steel.