News: Metal Coatings

Mastering Metal: How to Choose the Right Coating

By Jordan Scott

Architects and designers have several options when it comes to metal coatings. Powder, liquid and anodizing all involve different production methods and different aesthetics. It’s important to understand the benefits and limitations of each coating type to decide which is best suited for a particular project.

Anodized Aluminum

The anodizing process builds an oxidized layer on the metal in a controlled environment using an electro-chemical process. John McClatchey, vice president of sales and marketing at Atlanta-based Southern Aluminum Finishing (SAF), says the oxidizing process ends once the desired appearance is reached and then the metal is sealed. Anodizing can be clear or bronze, which requires a two-step anodizing process. The second step involves placing the metal in a tank and leaving it there until it achieves the desired darkness level. Achievable colors range from champagne to bronze to black. This process gives the metal a satin finish.

“Because you’re dealing with a natural substrate it’s analogous to staining wood. You’re at the mercy of the metal you’re using. Like paint, it can have color variations,” says McClatchey.

He explains that metal purchased from different mills will likely anodize differently.

“The best way to minimize color variation is to have all the metal come from the same mill and consecutive lots. If an architect wants to minimize color variation, SAF would have to order the metal ahead of time or hopefully have material on hand from the same lots … It can take six to 16 weeks from when the order is placed with the mill for the metal to come in,” says McClatchey, adding that this needs to be factored into the project schedule.

SAF sends architects range samples so they can see the potential color variation. McClatchey explains that the samples aren’t meant to give architects color options, but to show what the color difference could be if the target is in the middle of that range. He recommends that architects familiarize themselves with AAMA 611, Voluntary Specification for Anodized Architectural Aluminum, which explains the allowable color variations when anodizing aluminum.

Liquid Coatings

Scott Moffatt, market manager at Pittsburgh-based PPG Industries, says liquid paints are formulated with primers. These primers make the liquid coating resistant to corrosion.

McClatchey describes liquid coatings as putting a coat of plastic over the aluminum. SAF mixes the paints in-house and has technology to match mica and metallic paints. It uses a colorimeter by HunterLabs to measure the color variation to make sure it’s with-in the specification, AAMA 2605, Voluntary Specification, Performance Requirements and Test Procedures for Superior Performing Organic Coatings on Aluminum Extrusions and Panels.

McClatchey says the color variation is easier to control using liquid paint, which makes it attractive to architects who want to minimize that risk. He often sees liquid coatings specified for larger projects and anodizing specified for storefronts and similar applications.

While liquid coatings are prone to less color variation, McClatchey says metallic and mica paints tend to have more variation compared to earth tones. To minimize color variation, he says it’s important to have the metal painted at the same time by the same applicator.

Petersen Aluminum Corp. in Elk Grove Village, Ill., mostly works with coil coating for metal cladding using a fluoropolymer coating. Dave Landis, manager of technical services and field inspections with the company, has seen metallic and mica paints become increasingly popular in the past five years. Another trend on the rise is custom colors.

“Architects need to realize that there’s a cost and lead time impact when specifying custom colors. That’s usually not factored into their decision,” he says. “If a stock color can be available to a contractor in a four-week period then a custom coating could take upward of 12-14 weeks.”

Another point Landis emphasizes is that metal cladding requires a two-coat fluoropolymer, not a three coat.

“There is no warranty difference to the owner when an architect specifies a three- coat over a two-coat system,” he says, adding that this is frequently mis-specified because MasterSpec includes a three-coat option in its drop down format.

And if architects are specifying metal cladding within half a mile of salt or brackish water, the metal should be aluminum and not steel because steel will rust in that environment.

Powder Coatings

While liquid coatings have been used for the past 50 years, powder coatings are new to the U.S. market. Moffatt attributes this to its environmental benefits. Powder coatings don’t use volatile organic compounds or solvents. They also are recyclable and efficient, according to Moffatt, because they require only one coating.

Aside from the environmental benefits, powder coatings also provide a level of hardness not achieved through liquid coating or anodizing. This makes them suitable for interior applications where there is more foot traffic. However, powder can’t be used on coils due to the way it’s processed.

“If powder were put on a coil it would fly off,” explains Moffatt, who adds that powder is usually sprayed onto the metal.

Powder coatings are becoming favored by many of the large architectural firms in New York City, according to Moffatt, but around 90% of fabricators use liquid coatings.

He expects that as the market gains more powder lines, more powder coatings will be specified as high-profile projects are increasingly including powder coatings. However, these products do have their disadvantages.

“Liquid is superior in appearance compared to powder coatings. The mica and metallic paints are more wanted and prevalent. Liquid will always look better than powder at this time. Powder is 3-mil thick. Liquid is only 1-mil. Plus, the special effect pigments get lost in the thicker coat. Liquid is still king,” says Moffatt.

He adds that powder coatings also make the metal’s surface appear rough but this allows fabricators to create textured coatings more successfully compared to liquid coatings.

“Another thing you can do with powder that you can’t with liquid is create a speckle look using two different colors,” he says.

One thing architects should consider when specifying powder coatings is the environment surrounding the building. Powder coatings are often a one-coat system, but in areas by the coast or where road salt or fertilizers are used often, Moffatt says architects should consider adding a top coat to provide corrosion and color-fade resistance.

“A two-coat system will always outperform a one-coat system. In a normal environment it’s not as critical but in severe environments architects should look at this harder,” he says.

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