The Finishing Touch: Metal Coating Considerations

Powder, liquid Kynar® and anodizing coatings all involve different production methods and aesthetics. Glazing contractors have to be aware of these distinctions to order and install the products properly.

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 achievable anodized colors range from clear and champagne to bronze and black. The metal is placed in a tank and left there until the desired darkness level is achieved.

“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, who emphasizes that color variation is inherent to the anodizing process due to the differences in the chemical makeup of each piece of metal.

Metal purchased from different mills will likely anodize differently. That’s why it’s important to have all of the metal come from the same mill and consecutive lots if an architect wants to minimize color variation.

“The glazier typically will order the metal, but an architect would need to give the contract glazier enough time to place the order,” says McClatchey, adding that it can take six to 16 weeks for the order to arrive from the mill, if not already in stock, and this needs to be factored into the project schedule.

Liquid Coatings

Scott Moffatt, market manager at Pittsburgh-based PPG Industries, says that liquid paints are formulated with primers. These primers make the liquid coating resistant to corrosion. After priming, there’s a color coat followed by a clear coat. Liquid coatings are smooth in appearance. Mica and metallic flakes in the coating allow for special effects and create more sparkle compared to other coatings.

McClatchey describes Kynar liquid coatings as putting a coat of plastic over the aluminum. To minimize color variation, he says it’s important to have the metal painted at the same time by the same applicator, especially if the panels will be next to each other.

Petersen Aluminum Corp. in Elk Grove Village, Ill., works mostly with coil coating for metal cladding using a fluoropolymer coating. Dave Landis, manager of technical services and field inspections, says Kynar products have a 30-year paint finish warranty whereas anodized finishes do not. He explains that Kynar’s color uniformity from batch to batch is also warrantied, which means if a glazing contractor needs to buy more brake metal for a job, a Kynar product should appear the same in color if ordered from the same supplier.

However, Landis points out that glazing contractors need to remember that mica and metallic Kynar finishes are directional.

“Visualize a 4- by 10-foot sheet of metal. When we fabricate it, we fabricate it continually in the same direction. If it’s put in a different direction the panel will appear to be a different color,” he says. “When we ship it from the factory it has directional arrows on its protective film.”

Moffatt says that glaziers are starting to take on more of a color management role.

“Glaziers are calling us up directly and offering a service to manage color matching for architectural firms,” he says.

Landis says glazing contractors should know that there’s an increased cost and lead time impact when working with custom colors.

When brought into a project early enough, the glazing subcontractor can make architects aware of certain considerations that could help with the specification process.

“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.

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 coatings 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.

“Liquid is superior in appearance compared to powder coatings. The mica and metallic paints are more prevalent and desired. 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.

Jordan Scott is an assistant editor for USGlass magazine. She can be
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