Compatible Paint Systems over HDG
Some of my customers have asked what kind of paint is best for coating hot-dip galvanized steel. Is there a specific type of paint I should be recommending to my customers?
Making paint recommendations can be tricky. First, it could reflect poorly on you or your company if a recommendation turns out problematic. Second, it’s best to leave paint recommendations up to paint suppliers or specialists. Having said that, many types of paint have been applied to hot-dip galvanized steel over the years. Some patterns have emerged with specific types of paint in particular environments, some working better than others.
In the AGA publication, Duplex Systems, you will find a table (Figure 1, below) that discusses the compatibility of 16 types of paint or other barrier coatings that have been applied over hot-dip galvanized steel. While this information should not be looked at as a recommendation, it can be used as a resource to assist in choosing an appropriate type of paint or barrier coating to apply over hot-dip galvanized steel. However, the specific type of paint or other barrier coating that will be applied over the hot-dip galvanized steel is only part of the formula for ensuring a successful duplex system. The other part of the formula, and probably the most important, is the preparation of the galvanized coating prior to applying the paint. If the preparation of the galvanized coating is inadequate, it does not matter what type of paint is used, it will begin peeling off the galvanized coating sooner rather than later.
Clients asking for recommendations should also be made referred to SSPC Guide 19. Guide 19 addresses both the condition of galvanizing and the application’s service environment to narrow down types of paint that are compatible under these circumstances. Using Guide 19’s Table 1, you can easily see what paint types are recommended before exploring further options with a paint supplier. For example, on a deteriorated hot-dip galvanized coating in a chemical (<5pH or >10pH) environment, bituminous, coal tar epoxy, and epoxy paint types, would be recommended.
SSPC Guide 19 - Table 1 (Click here for the full SSPC Guide 19)
|Galvanizing Condition||Service Environment|
(pH <5 or >10)
|Coal Tar Epoxies||Yes||Yes||Yes||No||Yes||Yes||Yes|
|Inorganic Zinc Rich||No||No||No||No||No||No||No|
|Organic Zinc Rich||No||No||Yes||--||No||No||Yes|
|Micaceous Iron Oxide
Remember, even the best paint systems will fail if the galvanized surface is not properly prepared. So do yourself and your client a favor by emphasizing the importance of surface preparation when discussing paint. The AGA has published an instructional guide, Preparing Hot-Dip Galvanized Steel for Paint. The guide offers a detailed walk-through of the steps necessary to properly prepare a hot-dip galvanized coating for paint or other barrier coating. It is available for download in PDF form and can also be ordered as a CD or DVD with visual demonstrations.
Figure 1: Barrier Coatings Compatibility with HDG Steel
|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|
|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|
© 2023 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.