Metal paint systems – a primer

A diligent customer will leave no stone unturned, no concept misunderstood when purchasing a rural building. How will the wood be treated to ensure the longevity of below-grade structural members? At what distances will trusses be spaced? Will nails or screws be the fastener of choice?
What about paint systems? Any customer can be expected to grasp basic concepts like fade — bold brown siding that now looks like coffee with two creams is easy to spot. But can builders and consumers be expected to understand Hunter units, resin, or ASTM testing? “Most customers do not understand the differences between paint systems, and unfortunately can be misled,” says Ken Kellams of FBi Buildings.
Sometimes, this lack of understanding applies to the builder, who should be the one doing the teaching in the first place. “I think they may have a grasp, but I’m not sure if they understand how to use the information to their benefit,” says Ryan Parks of Central States Manufacturing. “For example, some builder might have an advantage of selling SMP versus regular polyester, but he doesn’t justify the cost to the purchaser because he doesn’t know how to communicate the benefits to his customer — so he goes back to the least common denominator, price.”
Customers who are looking for the cheapest possible product will always exist, and some companies will happily snatch their business. But as rural building technology continues to drive the market into higher-profile structures, the need for quality coatings that will best uphold a building’s appearance grows greater and greater.
“People spend hard-earned money on their building projects and want them to look good for the long term,” says McElroy Metal’s Ken Gieseke. “It does not matter if the building is in a backyard or a commercial structure, there is value in a coating system that has long-term performance characteristics.”
What follows is a primer on paint systems commonly used in rural building applications.

Performance criteria
There are dozens of criteria and tests paints are subjected to, including falling sand erosion tests, acid tests, adhesion after impact and bending, and salt spray and acid rain simulations. One of the more important characteristics is hardness, or scratch resistance, which is measured in pencil hardness. The test (ASTM D3363) uses pencil leads, ranging from 9H (hardest) to 6B (softest), to determine the hardest that will not scratch the coating. One pencil is the difference between two adjacent lead types.
Chalk, fade, and gloss retention are the three prime aesthetic measurements for paint systems. Chalk is the whitish powder that forms on the surface of a painted metal panel. It is primarily paint resin and pigment that has been degraded by UV and moisture. Chalk is measured on a scale of 1 to 10, 10 being the best, as outlined in ASTM 4214.
Color fade is largely the result of pigment degradation by UV radiation, but color loss also occurs when pigment is washed out of a resin that is pitting or dissolving. To distinguish it from chalk, fade is measured only after cleaning weathered panels of surface chalking in a specified manner. What appears to be a badly faded panel may in fact be merely chalked.
Fade is measured in NBS or E Hunter units, with 1 Delta E being the slightest color differential perceptible by the human eye. A change of 5 units is slight, and a problem to consider primarily when new and weathered material will appear near each other on a wall or roof. The best performing paints, generally Kynar 500/Hylar 5000, won’t change more than 5 units over 20 years or more.
Gloss retention, a feature of the paint’s resin, affects the tonal appearance and stain resistance of a coating. Specular gloss is measured, per ASTM D523, by an instrument mounted at an angle to the surface, typically 60 degrees, or at 85 degrees for low glosses. Measurements range from low sheen (less than 20 percent), semi-gloss (20-60 percent), glossy (60-85 percent), and full gloss (more than 85 percent).
Fade, chalk, and gloss are the most important characteristics measured in weathering tests. The most reliable performance testing data comes from South Florida test fences, which subject painted strips to humidity and high UV radiation. Test fences in different climates are sometimes used for comparison.

Paints have three basic components: resin, pigment, and solvents that evaporate during the curing process. Resins serve to bind and protect pigments and form a barrier over the substrate. Pigments create the color, and absorb the ultraviolet radiation that can destroy both the resin and the underlying primer.
Paints are usually identified by their resin type. This tends to obscure important variation in both the resin formula and pigment type. Many of the trade-offs in creating these systems are made to meet different price points for different needs. But paints should also be considered as complete systems, rather than a simple addition of better or cheaper parts.
There are three primary resins used for coating exterior metal products. Polyesters have traditionally occupied the low end of the market. They offer a hard, scratch-resistant finish and a wide range of gloss, but are prone to chalking when exposed to UV. Polyesters have been greatly improved since their introduction in the 1960s, however, with higher molecular weights and longer polymeric chains that create stronger, more UV-resistant bonds.
Silicone-modified polyesters create a middle ground between PVDFs and polyesters. Also known as silicone-protected and siliconized polyesters, SMPs use polymerized silicone to improve polyester’s chalk performance and gloss retention. Companies initially experimented with varying levels of silicone, and marketed high levels as superior to lower levels, but silicone became less important as the polyester resins themselves improved. Most SMPs now contain 30 percent or less silicone.
Polyvinylidene fluoride is acknowledged as the premium resin for coil coatings. Popularly known by its original trade name Kynar, PVDF is a kind of fluoropolymer, a family that includes Teflon and Halar. Key to these chemicals’ toughness is the bond between carbon and fluorine, the strongest possible polymeric connection.
PVDF resin has superior chalk resistance and gloss retention, as well as stain and chemical resistance. It is softer than SMPs and polyesters, however, making it highly formable without risk of cracking, but also relatively easy to scratch during transport or installation. PVDF is most durable when it makes up 70 percent of the resin; higher concentrations do not coat well, since the acrylic is important for dispersion during the coating process.
There are two general classes of pigments. Organic, or carbon-based, pigments are generally synthetic and relatively inexpensive to make. However, organics have fairly weak molecular bonds that are easily broken down by moisture, UV, and pollutants, and so are prone to fading. Inorganic pigments are those that do not contain carbon, and may be naturally occurring or manufactured. They generally offer good fade resistance, with the exception of carbon black. Many simple inorganics are metal oxides, such as the widely used iron oxide and titanium dioxide.

Paint systems
Four companies — Akzo Nobel, Valspar, BASF, and PPG — make all the PVDF-based coil coatings used domestically, as well as most of the SMPs and polyesters. These companies collect the various chemicals necessary for paints: pigments, resins, and various solvents, flattening agents, binders, and other ingredients.
Polyester systems have improved greatly over the years, but they remain the lowest tier for price and performance for coil-coated exterior metal. Scratch resistance is their strong suit. In years past polyesters were recommended for soffit, rainware, entry doors, and other applications without full sun exposure. Modern polyesters can take the sun better, but will never match the performance of PVDFs or quality SMPs.
Polyesters generally offer a broader color spectrum than PVDFs, but the brighter and darker colors, especially reds and blues, are most prone to fade. Their tendency to chalk ranges from fair to poor.
Despite long-term fade, chalk, and gloss issues, when formulated with stable pigments in light colors, polyesters offer a versatile and inexpensive coating that can look good for 10 years and more. Modern systems carry 10- or 15-year film integrity warranties, and boast of decent weathering performance results.
Silicone-modified polyester systems vary greatly in quality. Polyester quality outweighs silicone content in importance, but SMPs still outperform straight polyesters in chalk resistance. A more important difference is in pigment type and quality. Some formulations use the same ceramic pigments as PVDFs; others rely on simple inorganics or organics. Since the better resin does little to prevent an organic pigment from fading, paying for silicone is no excuse for going cheaper with pigment.
SMPs vary in gloss from 20-60 at 60 degrees (semi-gloss to medium gloss). Warranties for SMPs vary between 10-20 years and can feature impressive chalk, fade, and gloss retention promises.
Like polyesters, SMPs are harder than PVDF resins, making them more resistant to rough handling. They are also more brittle, and tiny fractures can form on bends during forming. Manufacturers have generally considered these microscopic fractures insignificant; however, at least one company, Everlast Roofing, warms its SMP coil before sending it through the roll former.
Kynar 500/Hylar 5000 systems, which are required to contain 70 percent PVDF, do not vary greatly between manufacturers. Since the paints carry 20- to 30-year warranties that allow for extremely little fade, the companies all use ceramics and appropriate inorganic pigments.

Choosing a paint
Most decisions about paint are compromises between performance and cost. Although price differentials have narrowed since PVDFs were introduced, Kynar 500/Hylar 5000 systems still cost roughly 15-35 percent more than SMPs, depending greatly on the latter’s quality. Polyesters typically cost 15-25 percent less than SMPs.
Color is an important consideration. Light colors reduce the appearance of chalk, and are readily formulated with inorganics. Some companies will offer polyesters in a light color range, but shift customers over to an SMP for darker ones.
With a chalk/fade advantage over polyesters, and a scratch/abrasion resistance advantage over Kynar, SMPs are the most widely-used paint system in the rural building market. One panel manufacturer reports that it is hard to sell Kynar/Hylar-coated products to the rural market, primarily because of SMPs’ price advantage. However, advances in pigment and resin technology have improved their performance.
As noted earlier, based on their chemical makeup, SMPs vary greatly in quality. “All SMPs are not created equal, nor are they necessarily applied with a uniform film thickness,” says Fabral’s Scott Bacon, whose company uses the Super Alurite 2000 paint system. Bacon says greater use of ceramic pigments have vastly improved SMPs’ long-term fade resistance, while improvements in resins have increased chalk resistance.
Substrate should not be forgotten when considering a paint system. “Although less expensive panels may be available and are manufactured with a galvanized substrate and a lower-quality SMP or polyester paint system, these should not be compared to a high-quality SMP with a Galvalume substrate,” says Tracey Boyle of Union Corrugating, whose company uses Akzo Nobel’s SMP product, Ceram-A-Star 950. “These panels may look the same now, but in five or eight years you will see a difference in the quality of the paint system.”
Central States recently chose to brand a private label, CentralGuard, that uses Valspar’s new WeatherX technology. Parks says this SMP has proven to be more resistant than its peers to chalk and fade within the first 10 years of a panel’s life cycle.
Polyesters still have a place in the rural building market, though few panel manufacturers are eager to discuss it. “Polyester paints are very poor, post-frame should not use these,” says Kellams. Others say polyester coatings are acceptable in use with light colors, such as white, or the most basic shade and shelter applications.
What about Kynar, the accepted top-performing coating which has thus far carved out a small niche in the rural building market? The high-end coating has made its way into high visibility applications like commercial buildings, banks, schools, and custom homes. Gieseke says Kynar is McElroy’s most popular panel coating, and two of the company’s largest customers, FBi and Stockade Buildings, use Kynar products almost exclusively.
Stockade’s Steve Keith says his company made the switch from siliconized polyesters to Kynar about 10 years ago. “Why did we switch? Simple: A better system with a better choice of colors and much less chalking and fading when compared to the SP,” Keith says. “The better chalking and fading ratio is really the major issue as far as I am concerned, especially with the brighter reds and blues and darker colors like evergreen we are using today.”
In a competitive situation when a customer prefers something other than Kynar, Stockade has a deduction from standard pricing so its dealers can convert to SMPs. Keith says this does not happen often. “The SMP is still a good paint system — it just allows for more chalking and fading over a 30-year warranty period than Kynar 500,” he says.
Conversely, Craig Walters of Walters Buildings argues that all panels will fade to some degree, and paying extra for a coating like Kynar is not worth it in most situations.
“It is difficult to see or notice a fade difference on panels,” he says. “Almost all panels, whether Kynar, SMP, or polyester can fail prematurely. The customer just wants the building to perform as it should. However, too many factors can cause problems — fertilizer, air pollutants, etc. Customers can be sold on the paint differences, but it is hard to justify cost difference when looking at green in different finishes — they all look the same.”
Walters believes paint companies over-promise what their systems can do, which can lead builders to over-warrant their packages. “We as post-frame builders continue to put longer and unreasonable warranties out there. The products are very good, but how often on any product do you see 30- or 40-year warranties?” he says. “It is better to take care of problems without the fine print, maybe a 15-year no-questions-asked warranty. If a paint company can’t guarantee the system for that long without a problem, don’t offer a 30-year warranty.”
The bottom line: educate yourself. As Bacon says, most reputable metal manufacturers take the time to ensure their customers have been properly trained on the performance characteristics of paint systems.
“Buyer beware,” says Joel Viechnicki of American Building Components. “Read your warranties and understand what chalk and fade mean.”

Editor’s note: This story was adapted from one that initially appeared in Rural Builder’s sister publication, Metal Roofing Magazine, in August 2003. Research for that story was conducted by former editor Ryan Reed.

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