That New-Paint Look: Keys to Formulating a Plan for Restoring Metal Exteriors

Article From Durability + DesignPhoto 1: High-performance factory finishes, such as the fluoropolymer coating applied to the exterior of this library in Arizona, provide a high degree of color and gloss retention.Photo by Timmerman Photography; photo courtesy of PPG Industries Inc.

Factory-applied finishes on roof panels, canopies, awnings, and curtain walls offer a superior combination of appearance and performance qualities, and typically consist of powder or liquid coatings that are thermally cured, or baked, after application.The product selected dictates the specific performance characteristics, but in general, baked factory finishes are durable and provide a long-lasting finish, with good color and gloss retention (Photo 1).

The performance of a given system depends on the local conditions of exposure, including the intensity of ultraviolet radiation, exposure to airborne pollutants such as acid rain, chloride exposures in seacoast and urban environments, time of wetness, and orientation after installation (vertical versus low-angle installations).

Depending on the exposure environment, manufacturers often provide 10- and 20-year warranties for the higher-end materials. The warranties address peeling and loss of color and gloss. It should be noted that warranties do not claim that the color and gloss will remain exactly the same, but that the change will not exceed certain thresholds. In other words, a change in appearance in an outdoor environment can be expected over time.

While painting to restore or freshen up a factory finish may be desirable, painting decisions should be made cautiously, as the application of additional material introduces a new risk—the potential for peeling, either between the field-applied coating and the factory finish, or within the factory system itself. Also, the field-applied material will not have the same “automotive look” of the factory finish.

When possible, it is best to maintain the factory coating for as long as possible through periodic washing and cleaning. Detergent washing and rinsing improves the appearance and is especially beneficial in areas that are protected from the cleansing effects of rainwater (Photo 2).

Cleaning the surfaces every six months prevents the buildup of detrimental chloride or acid salts, and removes debris that serves to increase the time of wetness. Mildew and fungal growth should be removed at the same time using proprietary products or a blend of household bleach, soap and water (a blend recommended by the International Zinc Association is 1 gallon of household bleach and 1 cup of mild soap in 5 gallons of water).

Photo 2: Small sections quickly washed to determine if cleaning, rather than repainting, is an option.Photos courtesy of KTA-Tator Inc., unless otherwise indicated

Even with a routine cleaning program, the appearance of the factory finish may still become unacceptable. A less-than-brilliant finish may be inconsistent with the image desired by the owner, or a change in the color scheme may be needed due to “rebranding” or when purchasing an existing property. In these cases, repainting may be the only alternative.

When repainting is required, it must be recognized that the labor and cost will not be the same as pressure washing a concrete wall and applying another coat of acrylic coating. The field application of coatings to a baked finish requires thorough surface preparation and frequently, the use of sophisticated coating systems.

Common factory-applied finishes

The finishes commonly applied to aluminum and galvanized metal in the factory are described in a number of industry standards, including the following.

  • ASTM A755/A755M-11, Standard Specification for Steel Sheet, Metallic Coated by the Hot-Dip Process and Prepainted by the Coil-Coating Process for Exterior Exposed Building Products.The standard indicates that the common finish coats used for the topside (exposed side) are polyester, silicone polyester, acrylic, fluoropolymer, plastisol, and polyurethane. The common coatings used on the bottom side are polyester and acrylic.
  • AAMA (Architectural Aluminum Metal Manufacturers Association) 2603, Voluntary Specification, Performance Requirements and Test Procedures for Pigmented Organic Coatings on Aluminum Extrusions and Panels.The AAMA 2603 performance requirements are the least rigorous of the three AAMA standards (one-year South Florida exposure). The standard is designed to identify products that will provide and maintain a good level of performance and general appearance. Coatings in this category are typically for interior use and include baked enamels (polyesters and acrylics).
  • AAMA 2604, Voluntary Specification, Performance Requirements and Test Procedures for High Performance Organic Coatings on Aluminum Extrusions and Panels. The AAMA 2604 performance requirements are in the middle range of the three AAMA standards (five-year South Florida exposure). The standard is designed to identify products that will provide and maintain ahigh level of performance and general appearance. Coatings in this category typically include silicone-modified polyesters and 50% polyvinylidene fluoride (PVDF).
  • AAMA 2605, Voluntary Specification, Performance Requirements and Test Procedures for Superior Performing Organic Coatings on Aluminum Extrusions and Panels. The AAMA 2605 performance requirements are the most rigorous of the three AAMA standards (10-year South Florida exposure). The standard is designed to identify products that provide and maintain asuperior level of performance and general appearance. Coatings in this category typically include 70% polyvinylidene fluoride (PVDF).

The standards address performance, with specific emphasis on color and gloss retention (Photo 3).

Photo 3: Industry standards address long-term color and gloss retention, which can be measured instrumentally in the field.

Examination of the existing finish

Before undertaking a repainting project, the generic type of the existing finish must be determined in order to specify a compatible overcoating material. If this cannot be obtained from project records, samples can be analyzed in the laboratory. If the original finish has already been repainted, the generic type of that material must be identified as well.

The adhesion of the existing coating must also be determined to make certain it is adequate to support the weight and associated shrinking and curing stresses of the new material. If the adhesion is poor, the application of the new coating could cause detachment of the factory coating from the substrate (Photo 4).

Photo 4: Overcoating material caused the factory finish to disbond from the original red primer and the galvanized substrate.

Suitable adhesion tests include ASTM D 3359, Measuring Adhesion by Tape Test, and ASTM D4541, Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers.

ASTM D3359 requires making a series of parallel scribes through the coating 1 or 2 mm apart, followed by another series perpendicular to the first to create a cross-hatch pattern. Pressure-sensitive tape is applied to the surface and removed. The amount of coating removed is rated according to an ASTM rating scale (Photo 5).

ASTM D4541 requires attaching dollies to the surface with an epoxy adhesion. A test apparatus is attached to each dolly and pulled from the surface. The pull-off strength of the film is reported in pounds per square inch. It is also important to note where the detachment occurs within the paint system. The manufacturer of the overcoating material can provide guidance as to the required level of adhesion for overcoating.

Photo 5: Good adhesion of the bare factory finish (right) and poor adhesion of thin, chalky coating previously applied to the finish (left).

Surface preparation

The quality and extent of surface preparation is critical in order for the overcoating material to perform, and it is often easier to achieve adhesion to a weathered and aged factory finish than to a new, glossy one.

In both cases, the surface needs to be thoroughly pressure washed as the first step, with care to protect or isolate electrical components. Pressure washing is usually accomplished at 3,000 psi with a fan tip and soap to remove dust, dirt buildup, bird droppings, grease, oil, and any other surface interference material that will prohibit proper adhesion of coatings. Higher pressures and oscillating tips can be used, but they may cause unintended damage to the film or the substrate.

Washing should be supplemented as necessary by scrubbing. Mold and mildew should also be removed if present, using a bleach/detergent blend. The initial cleaning should be followed with a fresh-water rinse.

It should be noted that in some cases, a thin waxy film may be present on the finish; this needs to be removed. If present, solvent cleaning rather than pressure washing may be required to cut the film. Localized peeling and deteriorated paint, oxidized aluminum, bare galvanize, and corrosion should be prepared by hand or power-tool cleaning. The goal is to remove the oxidation and to feather loose paint back to sound material. The transition between the bare substrate and the surrounding paint should be smooth and tapered, with no abrupt or uneven edges visible. The bare galvanize, steel or aluminum substrate should be roughened. A surface profile of 0.5 to 1.0 mil is typically sufficient.

Another alternative for preparing the bare steel and galvanize is the application of a phosphoric acid-based cleaning/etching solution.

If the existing finish is new and glossy, a critical step in the preparation process is the thorough and uniform de-glossing and roughening of the surface to provide an adequate “tooth” for coating adhesion. For small areas, this can be accomplished by hand or power sanding the surface, followed by rinsing to remove the dust created by sanding (Photo 6). Because factory finishes are typically only one or two mils in thickness, care must be exercised to avoid damaging or removing the coating when the goal is to simply de-gloss it.

Photo 6: Finish being de-glossed prior to overcoating. Small areas can be prepared by hand or power sanding.

For larger areas, cleaning and roughening can be accomplished by abrasive blast cleaning, provided the work area can be adequately isolated from tenants or the public, and the debris prevented from escaping the contained area. Requirements for cleaning and uniform roughening by dry blast cleaning are found in SSPC-SP 16, Brush-Off Blast Cleaning of Coated and Uncoated Galvanized Steel, Stainless Steels, and Non-Ferrous Metals.

If SSPC-SP16 is used, pressures should be reduced and the stand-off distance increased from that used for traditional blast cleaning to avoid warping the metal or removing the coating or galvanizing. Fine mesh abrasives should also be used.

Alternatives to traditional dry abrasive blast cleaning include sodium bicarbonate blast cleaning, sponge jetting and a low-pressure, low-impact blast cleaning process developed by Superior Coatings.

Sodium bicarbonate blast cleaning involves the use of compressed air to propel sodium-bicarbonate abrasive through a blast hose to a special nozzle where water is mixed with the abrasive. The water suppresses the dust and enhances the cleaning. The sodium bicarbonate remains aggressive enough to de-gloss the surface upon impact, before it dissolves. The surface must be thoroughly rinsed with fresh water to remove all residue of the sodium bicarbonate before painting.

Sponge jetting is another alternative to traditional blast cleaning. In this case, a special blast pot and urethane sponge media are used. As with traditional blast cleaning, the abrasive is conveyed through blast hoses using compressed air. The media consists of urethane sponge that encapsulates abrasive media such as glass beads, aluminum oxide, and plastic. The media is collected and reused. Standards for the media are found in SSPC-AB4, Recyclable Encapsulated Abrasive Media.

Superior Coatings developed Classic Blast, which is described as a low-pressure, low-impact abrasive blast cleaning process. The abrasive consists of walnut shells and 80 mesh Starblast® blended at a ratio of 20% walnut shells/80% Starblast® (Photo 7).

Photo 7: Special blend of walnut shells and Starblast® used for preparation of factory finishes.Photo courtesy of Superior Coatings.

Blast cleaning is accomplished at 40 psi, which is much lower than the traditional 90 to 100 psi used for blast cleaning. According to Jim Deardorff of Superior Coatings, the use of the special abrasive blend at lower pressures cleans and roughens the surface without causing unnecessary damage, since the cleaning process is more easily controlled and the abrasive blend is not as angular and sharp as other abrasives (Photo 8).

Photo 8: Aged, weathered coating cleaned with Superior Coatings abrasive blend with no damage to the galvanizing.Photo courtesy of Superior Coatings

Since the abrasive is not fractured at the low pressures, it can be reused multiple times. The efficiency of the process is also reportedly improved when using double venturi blast nozzles. After preparation, the surface is power washed to remove dust. In addition to buildings, the cleaning process is successfully used to prepare painted farm equipment and machinery for overcoating.

Field coating materials

The selection of the field overcoating material is based on the generic type of the existing coating, the service environment, and expectations for gloss and color retention. Selection of the wrong coating can lead to disbonding and aesthetics issues (Photos 9 and 10).

Photos 9 and 10: A low-quality alkyd coating was applied here as a touch-up paint. The color matched the factory finish at the time of application, but UV exposure quickly led to fading. The thin, chalky film also exhibited poor adhesion to the factory finish (see Photo 5).

The recoating materials will often involve the same generic coating type that is already present, assuming the existing performance is adequate. For example, if the factory finish is an acrylic, the repainting material will often be acrylic.

In other cases, the field-applied finish may be different than the factory system. For example, when the factory finish is a baked fluoropolymer, an epoxy bonding primer is typically applied to the surface, followed by one of a variety of finishes such as polyurethane or an air-dried fluoropolymer such as PVDF. The field-applied finish may also be fluorinated ethylene vinyl ether (FEVE), another type of fluoropolymer. Acrylic coatings can also be applied over most types of factory finishes.

Before selecting an overcoating product, the manufacturer of the field coating should confirm in writing that the product is suitable for the intended use.

Test patches

Before specifying a particular cleaning process or coating material, test patches (field mock ups) should be applied and evaluated. The test patches will confirm the extent of surface preparation required, whether the substrate can be prepared without damage or warping, and that the proposed coating system is compatible with, adheres to, and does not lift the existing coating. The test patches can also be used to confirm that the color, gloss and texture of the field-applied coatings are acceptable (Photo 11).

Photo 11: Test patches of various cleaning/painting combinations

The concepts for installing and evaluating test patches are found in ASTM D5064, Standard Practice for Conducting a Patch Test to Assess Coating Compatibility.

ASTM D5064 provides two exposure periods prior to testing. Long term is a minimum of six months. Short term ranges from seven to 14 days, depending on temperature. To be most effective, the test patches should be allowed to remain in place for six months in order to span seasonal weather changes.

When designing a test-patch program, consideration should be given to using separate patches for a range of methods of surface preparation in order to bracket the possible alternatives:

  • Method 1: Washing/detergent cleaning to determine if repainting without costly and time consuming roughening is possible
  • Method 2: Washing/detergent cleaning/solvent cleaning to determine if a solvent-soluble interference material is present (a significant improvement in adhesion over Method 1 suggests that such a material is present)
  • Method 3: Washing/detergent cleaning/mechanical de-glossing to determine if surface roughening is required in order to achieve satisfactory adhesion.

If it is expected that some type of blast cleaning will be performed for the actual project, but it is not practical to conduct blast cleaning for the test patch, sanding the surface will at least establish whether roughening is needed. What will be missing, however, is an assessment of the potential damage that blast cleaning might have on the existing finish or the substrate.

The candidate coating systems should be applied over each of the methods of preparation and examined for lifting, wrinkling, blistering, peeling, and quality of adhesion as described in ASTM D5064. At that point a final recommendation can be made.

A complex process, but…

Overcoating factory finishes is a complex process, requiring thorough, meticulous cleaning and often the application of sophisticated “industrial” coatings. In order to successfully overcoat a factory finish, a great deal of project-specific planning is required.

  • The existing coating type must be determined in order to select a compatible coating system.
  • The adhesion of the existing finish must be assessed to make certain it is a viable candidate for repainting. If the adhesion is poor, it may not withstand the weight and shrinkage/curing stresses of the overcoating material, resulting in detachment of the entire film from the substrate
  • An appropriate degree of surface cleaning and de-glossing/roughening must be specified to assure that the field-applied coating properly adheres to both the bare substrate and the factory finish. Typically, the newer and glossier the finish, the more extensive becomes the roughening requirement, but the surface preparation cannot be so aggressive that it damages the thin factory-applied coating or the substrate. Test patches (mock-ups) that combine a variety of cleaning methods with candidate coating systems can identify the potential problems that need to be addressed as part of the design.

 

Ken Trimber KTA-TatorKenneth A. Trimber is the President of KTA-Tator, Inc. and is directly responsible for the overall operation, performance, and success of the company. Ken has been employed by KTA since 1968, where he worked on a part-time basis until his graduation from college. With over 30 years of experience in the industrial painting field, he serves as a senior consultant and client liaison on many multi-disciplinary projects as well as a principal specification writer/reviewer. Ken is a NACE Certified Coatings Inspector Level 3 (Peer Review), is an SSPC Certified Protective Coatings Specialist.

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