Choosing the Right Coating for your Aluminum
The purpose of this article is to help you decide what type of coating to use — anodizing, paint, or powder coating when specifying architectural aluminum, whether the application is metal roofing, storefront, or curtainwall. The application and desired appearance dictate the finish selected. You may also go directly to How to Specify Anodized and Painted Finishes for more direct specification information.
Anodizing is a simple electrochemical process developed 100 years ago that forms a protective coating of aluminum oxide on the surface of the aluminum. The lifetime of the finish is proportional to the thickness of the anodic coating applied.
Aluminum oxide is a hard, durable, weather-resistant substance that protects the base metal. The coating may be colored by dying or may exhibit bronze tones through diffraction phenomena produced by the coating.
The coating grows from the base aluminum metal by this electrochemical process. The coating is integral to the metal and can not peel or flake. The structure of the coating is many small hexagonal pores, which are filled with a “seal” that hydrolyzes these pores to fill them with inert aluminum oxide.
Advantages Of Anodizing
- In general, anodizing is less expensive than painting, with the exception of coil painted products.
- Anodizing is harder than PVDF or FEVE. Anodizing is better for aluminum in high traffic areas where the coating is subject to physical abuse and abrasive cleaners.
- Anodizing cannot peel off. The coating is actually part of the metal.
- Anodizing gives aluminum a deeper, richer metallic appearance than is possible with organic coatings. This is because an anodized coating is translucent, and one can see the base metal underneath the coating. This translucence contributes to color variation problems, but anodizers are doing a much better job of controlling the amount of color variation than in the past. Computerized color matching with quantitative, objective color data is now commonplace in most anodizing facilities.
- Anodizing is unaffected by sunlight. All organic coatings will eventually fail due to exposure to ultra-violet light.
Good & Bad Anodizing
Anodizing that is improperly sealed has poor chemical resistance. Brand new anodizing with a thin coating thickness is nearly identical in appearance to an Aluminum Association Class I (very thick!) anodized finish, but thin (sub-Class II) anodized coatings are unsuitable for use on exterior curtainwall or metal roofing.
The advantage of a thicker anodic coating is its durability and longer life. The Achilles heel of anodizing is its chemical resistance. Eventually, the surface of an anodic coating may succumb to acidic pollutants in urban environments. Anodized surfaces, like other building components, must be protected from acidic attack during construction. The life expectancy of an anodized coating is determined by its thickness and the building’s environment.
After many years anodized surfaces may accumulate dirt and stains that look similar to chalking paint. This “chalk” can be removed with a mild detergent combined with an abrasive cleaning technique. A small amount of the anodic coating can actually be removed, leaving behind a renewed anodized finish which can last for another twenty years. This is why anodizers say their product is “renewable.”
Once an organic coating has failed, the only options are to re-coat the surface with another paint or replace the metal. When an anodized coating appears to have failed, cleaning often results in a renewed appearance.
Anodizing & the Environment
Anodizing appears to be compatible with today’s environmental concerns.
Though more research needs to be done to determine the total environmental impact of different aluminum finishes, from a finisher’s point of view, compliance with environmental regulations is easier than with other metal finishing processes.
There are anodizers positioned in every major urban area, and anodizing may help products qualify for LEED (Leadership in Energy and Environmental Design) points based on regional materials.
The chemicals from anodizing can be used by municipal wastewater treatment facilities. The aluminum sulfate from anodizing plants’ effluent actually improves the solids settling efficiency of some wastewater treatment plants.
Anodizing byproducts are often beneficial, like when aluminum trihydrate is often used as a fire retardant. Anodizing emits no ozone-producing solvents (VOCs), and there are no heavy metals involved in the process.
Paints & Powder Coatings
The performance of an organic coating (paints or powders) depends on the pretreatment, resin, and pigmentation.
With aluminum, the pretreatment is of utmost importance. This is why organic coatings for aluminum should be factory applied.
Resins are often the weak link in an organic coating system. Some resins, such as PVDF, have outstanding weather-resistance, while epoxy coatings are meant only for interior use.
There are many resins available for architectural use such as Acrylics, Urethanes, Polyurethanes, Polyesters, Silicon Polyesters, Polyester TGICs, PVDF, etc. Only a few of these coating systems will last for more than five years in exterior architectural applications.
The American Architectural Manufacturers Association (AAMA) has several excellent performance specifications for aluminum coatings, and AAMA 2605-17a is the most stringent specification for spray coated aluminum used in architecture.
An important aspect of AAMA 2605-17a is the requirement for actual ten-year exposure in South Florida. It is a good idea when evaluating different coatings to request the results of Florida exposure. PVDF coatings have been shown to pass AAMA 2605-13 in the widest variety of colors.
Remember that AAMA 2605 is only for spray coatings. The right specification for coil-coatings is AAMA 620. Because coil coated processes tend to be more tightly controlled, longer-term warranties are available.
PVDF coatings are usually formulated as 70% PVDF and 30% other resins, acrylic usually predominating. While 50% PVDF coatings are available, they do not meet AAMA 2605 in as many colors.
Chemical resistance and resistance to UV light are the strengths of PVDF coatings. PVDF has come to dominate the curtainwall and metal roofing markets because of its weather resistance, and because of the wide variety of colors available.
PVDF is produced by two firms: Arkema Inc and Solvay Specialty Polymers. Solvay is promoting PVDF under the trademark Hylar 5000® and Arkema Inc uses the tradename Kynar 500®.
Today, both Solvay Specialty Polymers and Arkema are providing PVDF resin to the four companies qualified to mix this special coating: PPG Industries, Valspar, Sherwin Williams, and Akzo Nobel.
There are also PVDF resins imported into the U.S. which are not Kynar or Hylar. This is why throughout this article, we have referred to these resins as PVDF whenever possible.
Advantages of PVDF Coatings
- PVDF is relatively chemically inert and will outlast anodizing in corrosive environments. Window washers can be less discriminating about the types of chemicals they use to clean a building. If extremely corrosive cleaners are used, however, even PVDF will show signs of damage.
- PVDF coatings offer nearly an unlimited selection of colors and are easy to manufacture in small batches.
- Coil painted sheet and does not craze as badly as an anodized sheet when fabricated.
- Color consistency is usually better than with anodizing. It is surprising, however, how many of our customers do not expect the slight variations that are seen in the color of painted products. Metallic metal flake coatings are especially prone to color variation. To improve color consistency, all the metal for a project should be painted by one applicator in one set-up.
Liquid paint is composed of pigment, resin, and solvent. Powder paint is simply pigment encapsulated in a powdered resin and is thus simply thought of as “paint without the solvent.”
Powder coatings and liquid coatings made from the same resin and pigment will have practically the same performance characteristics. For a given resin, the decision to use a powder or liquid coating is mainly a question of application technique.
The real advantage of powder is the reduction in air pollution compared to liquid coatings. When powders are cured in an oven, they emit no VOCs.
A disadvantage of powder is the large batch sizes that are typically required. The powder manufacturers are aware of this problem and a few of them keep colors in stock that they distribute in small batches.
Several companies manufacture exterior grade powders using a Polyester TGIC resin. Polyester TGIC powders are currently available in more stock colors than any other powder. Several companies stock hundreds of colors of Polyester TGIC powders. Polyester TGIC struggle with gloss retention compared to PVDF powders, but are widely used in Europe.
Some resins are more easily manufactured in liquid coatings, and some are more easily manufactured in powder coatings. A particular resin is usually manufactured in either powder or solvent-based coatings, but not both. Examples of this are epoxy which is predominantly a powder coating, and PVDF which historically has been manufactured as a liquid coating.
Many of the perceived advantages of powders over liquid coatings such as hardness and gloss are actually characteristics of the resin.
Powder coatings from most manufacturers are only available in large batches, and custom colors can be very expensive. This is because each batch of powder must be ground to order using costly grinding equipment. Solvent born colors will continue to maintain their niche in the market because of the ease with which small batches can be mixed. The ability to “mix and match” gives painters and their customers unequalled flexibility and ease of use.
During the last fifteen years, PPG, Akzo, and others have introduced AAMA 2605 fluorocarbon-based powder coatings. Most of these are based on FEVE resins, which are superior to Polyester coatings.
PPG has introduced the PVDF Duranar powder which offers not only AAMA 2605 performance. It is also practically identical in appearance to liquid PVDF coatings. We expect the use of AAMA 2605 powder coatings to increase.
SAF also applies epoxy-based powders for interior applications such as furniture. One note of caution about powders: they are prone to orange peel, and the coating may appear to be textured.
Environmental Considerations in a Coating Choice
Resins can theoretically be made in either solvent-born or powder coating formulations. Some resins are easier to manufacture and/or apply using solvent-borne formulations, but solvents cause air pollution.
The force of regulation weighs heavily on the construction industry, especially with solvent-borne coatings.
Solvent-borne coatings for long-term exterior architectural applications are mostly solvent-based where typically between 70 and 80% of each gallon of paint is evaporated during the paint curing process. Included in the fumes are hydrocarbons which are termed VOCs (volatile organic compounds) by the EPA, and are a precursor to ozone formation similar to automobile exhaust.
Ozone is a major component of smog and can cause eye, lung and throat irritation. Regulations such as the Resource Conservation and Recovery Act (RCRA) and the Clean Air Act will reduce ozone formation, although local action plans vary widely.
If Southern California is an indicator of future environmental regulations for the rest of the country, it may soon be illegal in the U.S. to spray many solvent-based coatings, including PVDF, without costly environmental equipment. This is driving up the cost of solvent-borne coatings.
Organic coating manufacturers may be forced to decide between recommending their applicators install costly pollution control equipment, or developing powder coating technology. Powder coatings produce no air pollution and are becoming full-fledged competition for anodized and solvent-borne coatings.
In order to qualify for the longest spray-coat warranties, the major PVDF coating suppliers still call for chrome-based pretreatment. A chrome pretreatment is required for long-term adhesion for exterior use, but it requires that applicators have extensive water pollution control equipment.
While this technology is well known and produces excellent adhesion results, the sludge from this process is a hazardous waste and is difficult and costly to dispose of.
In light of these new developments, specifiers may wonder what the best way is to specify a long-lasting painted finish. Our advice is to consult with your finisher, specify AAMA 2605, and to rely on the expertise of coating manufacturers like PPG, Akzo or Valspar who mix these coatings for the architectural market.
PVDF coatings have dominated the curtainwall and metal roofing markets because color consistency and color selection are more important than abrasion resistance. With PVDF, a specifier may choose any color he wants, and it is likely to be readily available since most applicators have the capability to mix their own coatings.
In most situations, PVDF coatings exhibit excellent color consistency. This does not mean that color consistency is perfect with paint, however.
Contractors must be careful to have their metal coated at the same time in the same place whenever possible to reduce the probability of color variation caused by different batches of paint or application conditions. Often there is severe color variation with three and four coat metallic PVDF coatings.
Architects’ objections to metallic paints are reminiscent of their objections to anodizing.
Care must be taken when applying touch-up paints because a perfect match is impossible between the factory-applied finish and a finish applied in the field.
Touch-up paints are a problem for both painted and anodized coatings. Touch-up paint should never be sprayed on, but touched on lightly with a brush. Touch-up paint will fade and chalk at a different rate than the underlying coating.
When a lively, translucent building exterior is desired, anodizing is a good finish for curtainwall. Architects should require color range panels from the anodizer. The installer should be aware of the variability expected from anodized panels. An installer may be able to sort some of the panels by elevation for the desired effect.
Sorting may also be required with metallic painted coatings. If a panel is outside the approved color range, the finisher should not ship it.
Importantly, it is also the installer’s responsibility to avoid installing metal that is not within range. When specifying anodizing for monumental use, be sure to specify SAFINISH (Aluminum Association Class I).
The reason for specifying SAFINISH anodizing (Class I) is to make an anodic coating as durable as possible. Coating thickness is the most significant indicator of durability for anodized coatings. Coating thickness for architectural use can be specified as either Class 1 (0.7 mils) or Class 2 (0.4 mils) per Aluminum Association DAF 45, but often there is no specification. The “cheapest thing” is then installed, and in a few short years, the finish is pitted, stained, and eroded.
Cheap anodized sheets are often sold with a coating thickness of 0.10 mils. A finish of this thickness also might be called a 200, A21, A22 or A24. While this coating thickness is suitable for many applications, specifiers should know that the integrity of this finish will not last more than a few years in exterior architectural applications. The expected lifetime is much less in coastal environments.
Coating thickness makes an even more important difference in the durability of colored finishes. Colors will fade more quickly with thinner coatings because they contain less of the coloring agent.
There is often enormous pressure to sell the “cheapest” product available. If a specification simply reads “Clear Anodized,” a 0.2 mil coating is the likely result. On the other hand, if a thickness of 0.8 mils is specified, the coating will last at least four times longer. Simply restated, the life of an anodic coating is proportional to its coating thickness, and a Class I coating is twice as thick as a Class II coating. This means that in most situations, Class I coatings will last twice as long a Class II on the exterior of a building.
As previously mentioned, anodizing can be renewed by cleaning. Anodizing can often be restored when it looks like it has failed, but when painted coatings fail, there is little that can be done short of repainting the entire building. Repainting a building is normally much more expensive than the cost of the original factory applied finish, and quality control is not good on a job site.
Both anodized and painted coatings require regular maintenance, something most building owners do not consider.
The parts of a storefront which receive the most abrasion from traffic should be anodized. Anodizing’s superior abrasion resistance means it will outlast paint on a door stile, kick-plate or push/pull bar.
On the other hand, painting aluminum framing materials above the doors adds a nice accent to a storefront. Sometimes, one will see aluminum doors installed in a mill finish to be painted at the job site. This is always a mistake.
Even if a field applied PVDF coating is used, while the resulting finish might last longer than conventional paint, its abrasion resistance would still be lacking compared to factory-applied PVDF or anodizing.
Anodizing will probably always have a place in the storefront market. Its abrasion resistance and cost-effectiveness for storefront applications are not likely to be matched by any organic coating. Specify SAFINISH coatings for the most durable, long-lasting finish.
The aluminum finish of choice depends on the application and is not merely a matter of personal preference.
Anodizing is best suited to storefronts, and anywhere else a rich metallic appearance is desired.
Anodized and Polyester coatings are best for storefront and handrails.
PVDF coatings are best suited for metal roofing and curtainwall components.
Anodized, PVDF and Polyester coatings could all be used on curtainwall, roofing and storefront applications with satisfactory results.
There are many options available for finishing aluminum, which is why it is such a popular construction material. The question of which finish to apply is not always an easy decision because of all the options available.
I have tried to be objective to help the specifier decide what type of coating system to use when coating architectural aluminum, whether the application is metal roofing, storefront, or curtainwall.
The conclusion I hope the reader draws from this article is to communicate your needs with your finisher or your finisher’s suppliers. With environmental regulations being the impetus behind many new developments, there will soon be even more options in the already vast array of finishes available for aluminum.
Kynar 500® is a registered trademark belonging to ARKEMA INC.
Hylar 5000® is a registered trademark of Solvay Specialty Polymers Inc.