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?
First, I would never recommend any specific type of paint to your customers because if they have a problem the first person they are going to blame is you. Since paint is not your specialty, leave the paint recommendations to your customers paint supplier. 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, with some types of paint 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 help your customer assist their paint supplier with choosing the best type of paint or barrier coating to apply over their hot-dip galvanized steel if they are looking for the increased corrosion protection offered by a duplex system. Remember though, 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 your customer uses, it will begin peeling off the galvanized coating sooner rather than later.
The AGA recently completed 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 and is available for download in pdf form. It can also be ordered with either CD or DVD with visual demonstrations. For more information on ordering your copy of this guide, please send an email to firstname.lastname@example.org
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 the zinc|
|Aliphatic Polyurethanes||Yes||If used as a top coat for a polyamide epoxy primer, it is considered a superior duplex system|
|No||Petroleum base is usually not recommended for use on galvanized steel|
|Bituminous||Yes||Used for parts 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 of 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 system w/base coat|
|Vinyls||Yes||Usually requires profiling, high VOC's have severely limited their availability|
|Powder Coating||Yes||Low temperature curing powder coating work exceptionally well over galvanized steel|
© 2019 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.