Spray Painting Galvanized Metal
Posted by diynovice on September 16, 2009
I recently made the mistake of promising my future wife that I could easily and cheaply spray paint a galvanized steel lantern we bought from IKEA. After three cans of spray paint and a couple of lanterns, I found out I was dead wrong. Now, I know a lot about metals, but I was never told that galvanized steel is difficult to paint. So, this post is about galvanized steel, and ultimately, the only spray paint that will work on it. For the readers who don’t care about the details, scroll down to the “**********” and continue reading.
Galvanized steel cannot be painted with normal alkyd-based paints, which almost all spray paints are based on (check the ingredients on the back of the can. If there is any “alkyd…” don’t even think about using it). Rust-o-leum is nice enough to put on the back of their spray paint cans to not use their spray paint on galvanized steel. However, beware, most spray paint companies do not include this warning.
First, what is galvanized steel? It is steel that has a zinc coating to increase the steels corrosion (rust) resistance. Most galvanized steel is created by a process called hot-dip galvanization and a common characteristic of these steels is their display of Spangle. Spangle is just a fancy word for visibly large crystalline grain size. Small amounts of lead and other impurities will increase the size of the spangle and make it more noticeable. You can see the spangle in the galvanized steel guard rail below.
Other galvanization processes used to apply zinc coatings usually do not produce noticeable spangle but instead have a dull gray finish. These processes are briefly described below (from Grip-Rite fasteners website):
Electrogalvanized – zinc coating applied to steel with an electric charge – offers limited corrosion resistance – typically applied to roofing nails
Mechanically galvanized– zinc applied by tumbling with powdered zinc and glass beads – provides slightly better corrosion protection than electrogalvaized steels.
Hot galvanized – zinc is applied through a heat treatment. Provides best corrosion protection behind Hot-dip galvanization.
How does the zinc coating work? The zinc coating provides corrosion protection by actively reacting with the atmosphere to form a thin, tough, inert layer of zinc carbonate to prevent the steel from rusting. The zinc coating will also provide cathodic protection if the underlying steel is ever exposed (such as by a scratch). While the zinc will always provide cathodic protection, it takes time for the zinc carbonate to form, and must undergo three transformations. First, the zinc rich coating will react with oxygen in the air to form zinc oxide. Second, the zinc oxide will react with oxygen and moisture to form zinc hydroxides. Third, the zinc hydroxides will react with oxygen, moisture, and carbon dioxide to form zinc carbonate.
Depending on the atmospheric conditions that the galvanized steel is subjected to, the time required for each of these layers to form will vary. Pure zinc will be present from 0 to 48 hours after the galvanization process; zinc oxides/hydroxides will form anywhere in 24 hrs to 2 years, and zinc carbonate will form in 8 months to 2+ years. Galvanized steel exposed to the elements will quickly form zinc carbonate (within 8 months) whereas galvanized steel located indoors and not exposed to the elements, can take more than 2 years to fully form the zinc carbonate layer. As the transformation advances, the surface will start to appear duller, but, the spangle of the surface will not be lost.
This is important for surface preparation, especially if you decide to brush paint and the steel is exposed to the elements. The zinc oxides and zinc hydroxides are not well adhered to the surface and can easily chip off. Zinc carbonate bonds well to the underlying zinc and provides an excellent painting surface. More in depth details on the surface preparation for painting can be found on the American Galvanizers Association (AGA) website (see links at the end). Rubbing the galvanized surface with a damp, lint-free cloth is most likely all that is required for the average DIY’er. Oils can be present on the surface from the manufacturing process, however, items for indoor/home use should not have these oils. Mineral spirits, turpentine, or vinegar can be used especially to remove any surface oils, however, these will leave a residue. Be sure to thoroughly wash the surface to remove this residue if you choose to clean with one of these.
Why can’t galvanized steel be spray painted? Alkyld-based spray paints will react with the zinc during any stage of the galvanized layer, in a process called saponification. The alkyd-base interacts with the zinc to form a soap at the interface. This will result in poor paint adhesion and paint peeling. Cold galvanizing spray paints will adhere to galvanized steel because of their high zinc content, however, top-coats of regular spray paints still will not adhere, and the colors of cold-galvanizing spray paints are very limited.
Many brush-on paints exists to cover galvanized steel, but spray paints appear to be non-existent. After much research, I finally found the solution. Acryllic latex will adhere to galvanized steel with minimal surface preparation. Therefore, the solution is Krylon’s H2O Latex spray paint. It is an acrylic latex that will not chemically react with the galvanized surface. [Krylon is one of the companies that does not include a warning against using their regular spray paints on galvanized steel. Don’t be fooled. All of their alkyd-based spray paints cannot be used on galvanized steel. And, I don’t think they realize that they have the only spray paint that can be applied to galvanized steel.] The spray paint costs about the same as any other spray paint and since it is a latex paint, it is more environmentally friendly. It is difficult to find in stores. I found it at Ace Hardware, but you can also find it from online retailers. This spray paint is less viscous (more watery or runny) than the average spray paint, so use multiple light coats to prevent the paint from running and pooling on your project (I learned this the hard way). DO NOT try to cover it in one coat. The paint dries in about 15 minutes, however, the paint will not fully cure for about 7 days. Also, do not use the Krylon H2O Latex primer. It is alkyd-based (so technically, not a latex spray paint) and will not adhere, just like any other spray paint.
Hopefully, this information will help any future DIY’ers with their projects. For more information, and more in depth explanation, visit the American Galvanizers Association’s (AGA) website at http://www.galvanizeit.org/ and check out their free publications on painting galvanized steel.
For general painting of galvanized steel, here is an excellent list of paints and their compatibility with galvanized steel. This table comes from “Duplex Systems: Painting over Hot-Dip Galvanized Steel” which is available on the AGA’s website.
Type (paint base)…..Compatible…..Comments
Acrylics …….Sometimes……If the pH of the paint is high, problems may occur due to ammonia reacting with zinc
Aliphatic Polyurethanes…..Yes…..If used as a top coat for a polyamide epoxy primer, it is considered a superior duplex system
Alkyds…..No…..The alkaline zinc surface causes the alkyds to saponify, causing premature peeling
Asphalts…..No…..Petroleum base is usually not recommended for use on galvanized steel
Bituminous…..Yes…..Used for parts that are to be buried in soil
Chlorinated Rubbers…..Yes…..High VOC content has severely limited their availability
Coal Tar Epoxies…..Sometimes…..Rarely used, only if parts are to be buried in soil
Epoxies…..Sometimes…..If paint is specifically manufactured for use with galvanized steel
Epoxy-Polyamide Cured…..Yes…..Has superior adherence to galvanized steel
Latex-Acrylics…..Yes…..Has the added benefit of being environmentally friendly
Latex-Water-based…..Sometimes…..Consult paint manufacturer
Oil Base…..Sometimes…..Consult paint manufacturer
Portland Cement in Oil…..Yes…..Has superior adherence to galvanized steel
Silicones…..No…..Not for use directly over galvanized steel, can be beneficial in high temperature systems w/ base coat
Vinyls…..Yes…..Usually requires profiling, high VOC’s have severely limited their availability
Powder Coating…..Yes…..Low temperature curing powder coatings work exceptionally well over galvanized steel
American Galvanizers Association. (n.d.). Duplex Systems: Painting over Hot-Dip Galvanized Steel.Retrieved from American Galvanizers Association: http://www.galvanizeit.org
American Galvanizers Association. (1999). Practical Guide for Preparing Hot Dip Galvanized Steel for Priming. Retrieved from American Galvanizers Association: http://www.galvanizeit.org
Avallone, Eugene A., Theodore Baumeister III, eds. Marks’ Standard Handbook for Mechanical Engineers. 10th ed. New York: McGraw-Hill. Pgs. 6-93 & 6-110.
Grip-Rite. (2008, june). Grip-Rite Fasteners Catalog. Retrieved from http://www.grip-rite.com/fasteners.asp
Malone, J. F. (1992). Painting Hot Dip Galvanized Steel. Materials Performance , 31 (5), 39-42: http://www.galvanizeit.org
Peeling – From Galvanized Metal. (n.d.). Retrieved from Sherwin Williams: http://www.sherwin-williams.com/pro/problem/problems/peeling_galvanized/index.jsp
Painting Galvanized Steel. http://www.galvanizingasia.com/pdfs/page65-69.pdf
Why does Rustoleum Rusty Metal Primer say not to use on galvanized metal? . (2008, May 8). Retrieved from Handy Man Club: http://www.handymanclub.com/Community/Forums.aspx?g=posts&t=38018