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John Sez:
By John Harris, Paint Chemist
Painting Galvanized Metal
Galvanizing steel creates one of the most effective coatings for delaying rusting. The galvanizing process involves dipping steel in molten zinc. When the zinc coating cools, the thin layer on the surface hardens into a crystalline or “spangled” finish. Before the galvanized steel leaves the factory, the product receives a bath in hot mineral oil. The purpose of the oil is to postpone the formation of the white powdery zinc oxide that forms on the surface when exposed to oxygen over time.
While protecting their product with this oil, the galvanized metal manufacturers cause problems for those of us who want to paint this building material. This oil needs to be removed before painting. The most effective method for removing the oil is with lacquer thinner and lots of rags or paper towels. The technique for cleaning oil from galvanized metal is to wipe an area with a lacquer thinner moistened rag, re-fold the rag to expose a clean area, wipe the wet area again removing the oil, change rags and repeat this cycle. Using only one rag merely transfers the oil in one area to another area. Lacquer thinner is better than paint thinner or mineral spirits, as some thinners may contain oil.
In the past, it was thought that using a mild acid to etch and remove any zinc oxide from the galvanized surface was helpful. Now, it is believed that leaving any of the white zinc oxide that has formed on the surface actually helps paint adhere.
After cleaning galvanized metal with lacquer thinner, a latex primer should be used. Oil or alkyd primers can chemically react with the zinc causing adhesion failure. Remember, do not prime or paint a surface that is too hot (above 90 degrees) or too cold (below 50 degrees), or that is in the direct rays of the sun.
Let us know of any coating problems you are having. We want to help you keep your paints on.
Belcaro Paint & Decorating Center, Inc. 5475 Leetsdale Dr., Denver, CO 80246
303-757-5435
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Belcaro website
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