Paint Selection Painting Hot-Dipped Galvanized Steel

Beyond the need for surface preparation, the paint itself must be compatible with the galvanized coating in order to create a successful duplex system. Many types of paint and paint systems have been used quite well with galvanized steel. However, some types of paint will not adhere adequately to galvanized steel, or will only do so under restricted conditions. In order to ensure a successful duplex system, it is important to find a suitable paint system with a first coat that is fully compatible with a zinc surface. The first coat serves as a “tie coat” or interface between the galvanized steel and the top coat.

To achieve a good interface, it is important to understand the characteristics of all the types of paint that will be used. Each individual formulation of paint exhibits unique characteristics that can affect its suitability for use with galvanized steel. Because of this, only individual paint manufacturers can provide specific guidance on the use of their products. Contact paint manufacturers for specific information regarding the suitability of paint systems for use on galvanized steel.

Below is a table that shows general compatibility between the specified paint system and hot-dip galvanized steel. Click on the individual paint name to get specific information regarding the paint.

Zinc-Rich Paints

Zinc-rich paints have long been recognized for their excellent paint adherence to both new and weathered galvanized surfaces. Zinc-rich paints have been used in the U.S. for more than 75 years and in Europe for well over a century. In a 1960s study by the American Iron and Steel Institute and the Steel Structures Painting Council, zinc-rich paint outperformed all other classes of paint. Significantly, at the nine year inspection in 1970, there was no loss of adhesion to the zinc surface.

With a high percentage of zinc in the dry film, these paints can synergistically combine with the corrosion inhibitive properties of metallic zinc. The zinc dust in paint is integrated with organic binders. These binders allow the zinc particles to remain in contact with each other so the zinc paint can provide cathodic protection.

Zinc-rich paints are an accepted method of repairing damaged galvanized coatings according to ASTM A 780. Zinc-rich paints containing at least 65 percent zinc meet the specification designations. They are widely used for touch-up and repair of damaged galvanized coatings because of their relative ease of application.

Although zinc-rich paints are useful as primers to gain surface adherence, they are also satisfactory as a finish coat when a neutral or matching gray color is desired. These paints can be used alone, but for a more attractive finish a top coat is often employed. While most top coats are easily used, some with very strong solvents may result in a lifting of the primer. Successful top coats include polyvinyl, acrylic latexes, polyurethanes, and polyamide cured epoxies. Specific manufacturer’s recommendations should be followed for application and top coating.

Acrylics

Acrylics are single-component coatings, generally applied over a primer due to thin film build. A wash primer may be used with these paints, or they may be applied directly over the hot dip galvanized surface. If the pH of the paint is high, problems may occur due to ammonia reacting with the zinc. Acrylics provide exceptional gloss and color, combined with an extremely durable finish.

Aliphatic Polyurethanes

This is a two-component, high performance system generally applied over a polyamide epoxy primer or a wash primer. These polyurethanes have superior weathering and chemical resistance characteristics with good adhesion, as well as an enamel-like finish. This system requires strict attention to application procedures. If top coating is necessary, a light abrading or roughening of the surface is generally required.

Alkyds

In moist areas, zinc will produce an alkaline surface causing alkyds to saponify, resulting in premature peeling and flaking of the paint system despite initial satisfactory adhesion. Due to this chemical incompatibility with zinc, alkyds are very difficult to use on galvanized surface unless the paint is specifically formulated for using over galvanized steel. Contact the paint manufacturer for more specific recommendations on using an
alkyd paint system.

Asphalts

Asphalts are generally petroleum based products that are not recommended for use on galvanized steel.

Bituminous

These types of paints are thicker than conventional paint systems. As they are coal tar products, unlike asphalts, they can be used with galvanized steel. Bituminous paints are often used over galvanized steel that will be buried in soil.

Chlorinated Rubbers

Although difficult to apply, chlorinated rubbers are fast drying and provide good protection for exterior exposures and chemical resistance to acids, alkalis and most gases. However, they chalk readily and need a high surface profile for good adherence. In addition, their high VOC content has severely limited their availability and end use.

Coal Tar Epoxies

These types of epoxies are rarely used over galvanized steel. They provide outstanding resistance to acidic conditions in splash and spill areas. However, they are difficult to apply and require brush blasting or a wash primer to adhere to galvanized steel. Coal tar epoxies also are often used over galvanized steel that will be buried in the soil.

Epoxies

In most cases, epoxy-esters and epoxy-amines are not generally recommended for use directly on galvanized steel as they are typically high stress materials and may react with the zinc in certain environments, however, epoxies do have some limited success if the paint is specifically formulated for using over galvanized steel. Contact the paint manufacturer for more specific recommendations on using an epoxy paint system.

Epoxy-Polyamide Cured

These epoxies generally have superior adherence to any type of galvanized surface. Because they are not resistant to sunlight, they are typically used as a primer or for corrosive interior applications. A galvanized steel/polyamide epoxy primer/aliphatic urethane top coat system is considered to be a superior high performance duplex system.

Latex-Acrylics

Fast drying and water-based, latex-acrylics have great adhesion, durability and weathering characteristics. This system is often top coated with itself and is suitable for new and weathered galvanized steel. These paints have the added benefit of being environmentally friendly.

Latex Water-Based

This type of latex paint is also fast drying and weathers well, but takes time to cure before it provides acceptable adhesion and abrasion resistance. Therefore, these paints are not recommended for shop application. Adhesion and abrasion resistance improve with time (two to four weeks).

Oil-Based

Oil-based paints are poorly suited for use directly over galvanized steel. These paints are easy to apply, but have unsatisfactory chemical and solvent resistance. They are not generally used over galvanized steel as the oil can react with the alkalinity of the zinc and saponify in moist or humid environments. (see alkyds)

Portland Cement in Oil

These single package paints incorporate Portland cement as part of the pigment. They have outstanding adhesion to galvanized steel, but are often top coated since they do not weather as well as other coatings and may yellow with age. Occasionally, they do become brittle with time, so formulas with special resins designed to preclude embrittlement may have better success.

Silicones

Silicone’s suitability for use directly over galvanized steel is poor, and therefore it is not widely used. However, silicone is sometimes employed in high temperature applications where it develops a cross-link silicate that prevents oxidation of the zinc coating. Silicone-alkyd compounds typically do not perform as well as silicon-acrylic compounds.

Vinyls

Vinyls have exceptional resistance to acid and alkali environments and can be supplied as either a thin film needing top coating or as a high-build coating. As a rule, vinyls exhibit only fair adhesion and should be assisted by the use of surface profiling such as a sweep blast or a wash primer. Vinyl acrylics have a great glossy finish with good color retention. High VOC levels have limited their availability and use in certain areas.

Only a partial listing of available paints and paint systems has been provided. The paint manufacturer can provide more thorough information about the compatibility of specific systems with galvanized steel. Always consult the paint manufacturer prior to painting galvanized steel. Different physical and chemical characteristics of the same types of paint may have varied reactions with a galvanized surface. The paint manufacturer and the galvanizer can assist in the creation of a successful duplex system.

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