November 1, 2016
by Ed Telson
Even those best-designed, well-constructed buildings degrade if the appropriate exterior coating and finishes are not applied and maintained in a timely manner—this is especially true for demanding environments. Fortunately, there is a wide-ranging array of high-performance architectural coatings available today that can not only safeguard a building’s value, but also create dramatic visual appeal.
It is important to differentiate in the architectural world between the terms, ‘paints’ and ‘coatings.’ The definitions can often blur and be confusing, especially as the industry continues to evolve. The term ‘paint’ can refer to any coating, but is more typically marketed for aesthetics and the general DIY market. The term ‘coating’—especially those deemed ‘functional’ or ‘protective’—applies to products that have likely been tested to meet independent, and often higher, standards. (A third term, ‘finishes,’ should not be confused with paints or coatings, even though all three are often incorrectly treated as being interchangeable. Solids make up the bulk of finish products, and they are applied with a trowel in contrast to coatings and paints that rely on spray units, rollers, and brushes. Some finishes may offer protection from the elements depending on build and quality, while others may be purely decorative.)
Conventional exterior paints do not usually have the same protective qualities as coatings; rather, they are typically more prone to soiling over time and incur mildew and algae caused by weather and wind drift. Ultraviolet (UV) rays from the sun may further accelerate the deterioration of paint. Coatings, on the other hand, are typically specified as a prescriptive system with reference to millage build and application steps designed to achieve a higher level of performance than mere paint. The difference between paints and coatings can be seen in product data sheets, which list test and performance data.
Higher millage, balanced with superior resin performance, can be used as another way to characterize the difference between paints and protective and functional coating systems. When comparing millage, acrylic paints are typically applied in a dry film thickness (DFT) of 0.06 to 0.08 mm (2.5 to 3.5 mils) and contain solids in the 35 to 40 percent range. In other words, 60 to 65 percent of the solvent used to apply the product evaporates.
Architectural protective and functional coatings can range from 53 to 70 percent solids, or even higher. This leaves a DFT between 0.25 and 0.8 mm (10 and 30 mils) on the surface, with only 30 to 47 percent of the product evaporating. In this way, the millage specified by the manufacturer in product data sheets provides an indication of the optimal performance standard for a given protective architectural coating system. With a higher percentage of solids, such coatings offer both decorative and protective characteristics. The industry trend has been to utilize these higher-build acrylics on claddings and substrates for a longer cycle and protection.
Coatings come in a wide variety of choices ranging in quality and system capability—from providing more decorative, short-term benefits to superior protection and durability. Again, an evaluation of product specifications and testing data is the best indicator of probable performance of a coating product and system. Building owners seeking maximum protection for a façade should assess its condition and need for repair before carefully evaluating the available coating solutions.
Properties to look for in coatings
While every job is different and coatings vary based on the type of cladding and other variables (e.g. regional climate), the right product can offer protection against UV degradation, heat, salt, wind, humidity, dirt, and rain. It should allow a substrate to ‘breathe’—that is, be permeable and enable water vapor to pass through the envelope as it seeks equilibrium between exterior and interior humidity levels. A coating with ‘permeance’ can help resist blisters and reduce the odds of mold development in the wall cavity caused by moisture from vapor migration.
A coating system must also repel water externally, resist cracking, and help prevent corrosion in steel embedded in the substrate, while remaining safe for workers and the environment. Finally, ideal coatings should resist fading and also be easy to clean and maintain to help sustain the life cycle of the structure and thus reduce maintenance costs.
Architects and builders specifying a job should seek out and compare coatings with the tested characteristics listed in the following paragraphs.
There are coating products that are resistant to wind-driven rain whose absorption of water on vertical surfaces helps deter moisture intrusion and its ill effects. ASTM 6904, Standard Practice for Resistance to Wind-driven Rain for Exterior Coatings Applied on Masonry, measures the capacity of high-build coatings to provide an impermeable barrier to wind-driven rain due to the absence of voids such as pinhole entryways for water to enter a building. Test results should be available on product data sheets to allow for comparing applications.
As mentioned, vapor permeability allows a structure to ‘breathe,’ and these permeable coatings are less likely to blister due to water vapor movement. They help mitigate the effects of condensation that can lead to unhealthy interior environments. Permeance is a coating property that permits or restricts movement of water vapor. ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials, and ASTM E1653, Standard Guide for Specifying Dynamic Characteristics of Optical Radiation Transmitting Fiber Waveguides, should be reviewed for recommendations on evaluating water vapor permeance when comparing coating systems.
One should look for elongation properties that not only stretch, but also recover so the coating remains intact over cracks caused by thermal changes while also effectively camouflaging surface imperfections. ASTM D412, Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers−Tension, or ASTM D2370, Standard Test Method for Tensile Properties of Organic Coatings, measure elongation, crack-bridging, and tensile strength, can be compared when testing parameters are known.
Elastomeric coatings, or what the industry refers to as ‘high-build’ products, can resist wind-driven rain and the absorption of water on vertical surfaces. However, it is important to point out coatings are not a cure for structural defects that can lead to moisture intrusion. Possible defects need to be assessed and corrected before any coating application.
Technological advances in the coating industry have been formidable in recent years, yielding a new landscape of products with remarkable qualities. Some of these high-tech coatings can create a durable surface that both beads water and sheds dirt, helping keep walls clean and dry to extend a building’s life cycle and aesthetics.
Bond and tensile strength
This can help ensure bonding with the substrate, whether brick, concrete, stucco, or exterior insulation finish systems (EIFS). The old adage is coatings are only as good as that to which they are attached. Tensile refers to the maximum tension a coating can withstand without tearing it apart. ASTM D4541, Standard Test Method for Pull-off Strength of Coatings Using Portable Adhesion Testers, measures the pull-off of a coating and ASTM C297, Standard Test Method for Flatwise Tensile Strength of Sandwich Constructions, provides data to compare bond and tensile strength.
Volatile organic compound (VOC) content
There are coatings with low or no volatile organic compound (VOC) content. Indeed, all coatings should be easy to apply, safe, non-toxic, environmentally responsible, and compliant with environmental codes and standards.
Driven both locally and nationally, VOC standards vary. The strictest standards are set by the South Coast Air Quality Management District of California (SCAQMD), which continues to revise and update its VOC rules. Many locales, and even the U.S. Environmental Protection Agency (EPA), have lower standards. The trend is for increased environmental protection, with manufacturers becoming more proactive and adopting the philosophy of building with conscience in anticipation of more stringent requirements.
Possessing suitable pH resistance allows a coating to be applied in situations in new construction where high alkalinity may exist. High alkalinity is caused by cementitious surfaces (generally in new construction) where moisture has not sufficiently migrated out of the masonry. This generates salts, resulting in a high pH above ‘7.’ This can result in premature coating failure and efflorescence, affecting the coating’s polymer. Construction schedules and nature do not always cooperate to allow adequate time to achieve a naturally lower pH. Testing using pH pencils or strips is important when choosing a system to achieve durability.
Having a high carbon dioxide (CO2) diffusion resistance helps protect embedded steel in concrete structures from corrosion. Carbonation lowers a concrete’s pH, which reduces the corrosion protection it provides embedded steel. Manufacturers report both carbon diffusion resistance and the diffusion resistance coefficient in their data sheets (when appropriate) for coating systems. It is important to compare the same values between products when looking at data sheets. EN-1062 standard is generally used as the testing standard when comparing values.
Application of primers and sealers can promote uniform substrate absorption, which helps maintain proper DFT and addresses high pH where and when needed. The proper base coat can also provide resistance to efflorescence and help bond the coating to the finish. Additionally, a primer can reduce overall costs by helping to achieve consistent topcoat coverage.
Textured versus smooth
Applying textured, acrylic-based coatings over prepared vertical concrete, stucco, or masonry surfaces can help hide minor surface imperfections.
Accelerated weathering testing
Accelerated weathering testing helps benchmark coatings against one another when exposed to simulated weathering conditions. These tests are run in in chambers with intense light sources (e.g. carbon arc, fluorescent ultraviolet [UV], or xenon arc) where condensation or water is used as part of the test cycle. ASTM G155, Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Non-Metallic Materials, involves accelerated weathering tests that report the number of hours and condition of the sample with a generally accepted minimum threshold of 2000 hours. The duration of the test does not correlate directly to calendar time; it cannot be translated into a certain number of years. The test is best used as a benchmark to compare products.
The ability of a coating to exhibit little or no mold growth at higher relative humidity (RH) levels is important for the maintenance of any building envelope. It is generally reported under ASTM D3273, Standard Test Method for Resistance to Growth of Mold on the Surface of Interior Coatings in an Environmental Chamber, where the product is exposed to conditions that can lead to mold growth.
Color options and performance
Coatings that contain high-performance pigments and colorants will better resist premature fading. Manufacturers usually design both the base of the product and colorant systems to test them together on panel farms in environments such as Florida and Arizona where UV degradation and weather vulnerability are greatest. This data is provided to indicate relative performance, as well as the accelerated weathering tests previously mentioned.
Technological advances in the coating industry have been formidable in recent years, yielding a new landscape of products with remarkable qualities. Visible light catalyst (VLC) technology, for example, incorporates odor-neutralizing properties that can improve indoor air quality (IAQ). When VLCs are exposed to natural or artificial light sources, they degrade organic compounds to eliminate odors in the air. This phenomenon becomes even more important for air quality as buildings come under stricter codes for reducing air exchanges in a building as a trade-off for efficiency. As an easily applied single-component acrylic latex, such products require no special equipment or application method, and can be used on both interior walls and ceilings.
Overall, the optimum coating and its system is one that has functional qualities as well as aesthetic appeal and is appropriate for the specified cladding or substrate. Other considerations include ease of application and versatility. Some coatings offer multiple features as an all-in-one package, and may be less labor-intensive and more cost-effective. However, the primary consideration when selecting a product or system should be whether a coatings solution is prescriptive based on the type and condition of the substrate and the desired level of protection.
Which coating is right for your cladding or substrate?
Coatings play an integral part in maintaining and protecting the cladding and overall structure of a given building, but assessing the needs and requirements of a specific building and its substrate is always the first step in developing a professional specification for a coatings project. Choosing products based on comparison of data and capabilities from manufacturers’ product data sheets should be the main consideration when selecting a system that meets project needs in terms of function, durability, and aesthetics.
Careful review of product testing is crucial to substantiate performance, as is careful review of the application requirements in preparing the surface to accommodate the appropriate film thickness. When application standards are met, the coating system will deliver on aesthetic expectations and perform for its anticipated life cycle.
Builders in the past were told to back-prime wood cladding, and caulk and seal over all overlaps—only to discover that water vapor and condensation created damage. This necessitated a change of practice so horizontal overlaps were kept open instead of caulking every seam. Today, the best coatings for wood cladding are acrylic and permeable.
When wet, concrete masonry units (CMUs) can retain moisture and create problems as structures seek equilibrium requiring internal dehumidification. If left unaddressed, this can result in premature exterior coating failure for systems that cannot handle the transfer of moisture. The best coatings to combat this are also acrylic and permeable.
Stucco or finishes are sometimes applied without the proper air and moisture barriers in place, with the false expectation paint alone protects the building envelope. This can lead to expensive repairs, so one should take into consideration coatings tested for weather resistance, elongation, and construction of the entire envelope when specifying a system.
The best coating is a high-performance 100 percent acrylic topcoat that provides protection against sun, salt, wind, and corrosion. It should be a vapor-permeable product with high solids content, which will allow moisture to pass within the building cavity, resisting blisters and mold.
Economic and environmental benefits
Today’s innovative coatings can contribute to cost savings in the long run. Whether new construction or remodeling, choosing the right coating can preserve the value of buildings by serving as a permeable, durable, attractive layer of protection that extends the life of a structure by preserving it from the weather.
With self-cleaning and fade-resistant properties come lower maintenance costs, less cleaning, and longer recoat cycles. Labor costs always account for most of the expense in any coating application, while materials are typically the smallest portion of the coating project. Specifying a functional as well as decorative coating system typically results in a minor incremental increase in material cost, but when it is spread over the building life cycle, it pays dividends in performance and protection.
The right coating system also contributes to cost-saving efficiencies on the jobsite, typically in labor due to higher solids and simplification of the design and specification process. Finally, many coatings today also offer energy-saving properties, which reduce carbon emissions and help protect the environment.
Ed Telson has been in the coatings industry for more than 30 years consulting with, and holding positions in, management and sales with national and regional paint manufacturers, as well as having his own contracting firm. He currently holds the position of coatings segment manager for Sto Corp. U.S. Telson’s experience ranges from development to application of architectural and industrial coatings. He grew up as the son of a painting contractor and owner of a paint store in South Florida. Holding a NACE International Level 2 Certification for Coatings, Telson is a Stetson University graduate with a bachelor’s degree in business administration. He can be reached via e-mail at firstname.lastname@example.org.
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