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Metal Roof Coatings: Polyester vs. SMP

In my previous article I addressed the topic of metal roofing substrates and expanded on the topic of Galvalume® and other coatings that are used as substrates. Now I want to visit at another related topic that affects metal roofing performance and aesthetics, the paint.

A popular discussion among metal roofing channels when talking about paint systems is the debate between Kynar®/Hylar® (PVDF) and Silicone Modified Polyester (SMP). A less discussed and perhaps even more important topic is the major differences between polyester and silicone modified polyester (SMP) coatings.

When we talk about SMP, poly and Kynar®/Hylar® as "paint systems" or coatings we're actually referring to the resin. Resin is a solid that is convertible into polymers. It acts as a binder to hold pigment particles together and provide adhesion to the surface being painted. Resin also serves to provide the film its gloss as well as offer protection against abrasions and scratches.

The difference in performance and aesthetics between poly and SMP resins is staggering. Silicone-modified polyester based systems boast chalk and fade warranties of up to thirty years. These systems provide color and gloss retention and weather resistance that rival that of premium Kynar®/Hylar® PVDF coatings. Poly resins are unable to come anywhere close to this kind of performance and may begin breaking down in as little as three to five years. We're going take a look at the properties that make up both polyester and SMP coatings to get a better understanding of this drastic contrast but first we'll lay some ground work with an overview on pigments.

Pigments: Inorganic vs. Organic

Pigments are what give your metal roof its color. When pigments are blended with resins and mixed with a solvent it creates what is known as "paint". The pigment provides color and hides the primer on the substrate. The resin binds the collective coating to the substrate and provides weather resistance and protection from the environment. Just like there are different forms of resin, there are different forms of pigmentation. Pigments are either organic or inorganic (they can also be classified as "ceramic" which is a complex subclass of inorganic pigments). Organic pigments are low quality pigments that offer limited durability. Of course this means that such pigments are less expensive and as a result are often mixed with low grade polyester resins to formulate a cheap paint system at the cost of performance. Inorganic pigments are synthetic or naturally occurring without containing carbon compounds. The majority of colorants in this class provide excellent performance when mixed with quality resins. Ceramic pigments are complex inorganic pigments made from mixed metal oxides synthesized at molten metal temperatures. These pigments offer unparalleled resistance to heat, weather, light and chemicals.


Polyester or "poly" resins are unsaturated synthetic resins. These resins are formed by a reaction that takes place between dibasic organic acids and polyhydric alcohols. Poly resins blended with organic pigments give us the poly paint systems used on some soffit or interior applications as well as some "secondary" or "economy" metal roofing systems.

Poly resins have historically been used on soffit and industrial applications that don't require rigid weather resistance properties or aesthetics. Unfortunately, the pursuit of an easy sell with low cost materials has driven some manufacturers to use poly resins in residential and architectural applications. When poly resins are mixed with organic pigments the result is an inferior paint system that will degrade rapidly. In as little as three to five years, gloss will degrade giving way to chalking and drastic color fade. Bright reds will turn pink. Deep blues will turn yellow. The fade in this paint system is often inconsistent resulting in an unsightly "splotchy" effect across the roofing system. You will be able to run your finger across the surface of the roof and literally feel and see the chalky residue on the panel surface. The worst part of this story is that the cost savings represented by such a subpar system is minimal. Buyers can expect to save around twenty cents per linear foot with poly coatings versus SMP coatings. Many times buyers purchase systems with these low quality coatings unaware that they are indeed subpar. Crafty warranty language for some poly coatings can give the impression that the paint system will last as long as ten to twenty years when in reality the fine details of the warranty allow for flaking, crazing, fading and chalking. Suppliers that choose to coat their materials with low grade poly coatings often couple these coatings with a subpar AZ35 or AZ40 substrate. This creates a significant problem for the integrity of the metal panel. Paint systems are permeable which means by nature they have a cycle of absorbing and releasing moisture, another reason a quality substrate is so important. After numerous cycles of moisture absorption against an inferior substrate the metal itself will begin breaking down under the coating resulting in material failure. Quality matters.

Silicone-Modified Polyester (SMP)

Silicone-modified polyester, or SMP, is a popular high quality resin used for metal roof coatings. SMP resins are created by blending polyester with silicone intermediates and ceramic pigments. This formulation results in a superior color and gloss retention as well as increased weather resistance when compared to poly resins. As a general rule the higher the silicone content, the better the performance. However, recent advancements in ceramic pigment technology have allowed lower levels of silicone (lower than 50% silicone) to be used in SMP formulations without sacrificing performance or color/gloss retention. WeatherXL by Sherwin-Williams is an example of such technology.

Reputable paint manufacturers like Sherwin-Williams or Akzo-Nobel use only ceramic and select inorganic pigments in their SMP formulations. These coatings approach the durability and efficiency of premium PVDF coatings and boast comprehensive fade warranties as extensive as thirty years. The nature of silicone-modified polyester coatings allow it to retain color for decades with minimal fade that is uniform across the exposed surface. In addition to the durability and aesthetic performance of these coatings, they also possess energy efficient properties that poly coatings lack called solar reflectance. Different colors have different solar reflectance ratings, typically the lighter the color, the more efficient. The efficiency lies in the coating's ability to reflect a portion of solar radiation (hence the term "solar reflectance") away from the roof surface as opposed to absorbing the energy and thus battling against the structure's HVAC system. The efficiency of solar reflectance in SMP coatings is so abundant that roofing systems with these coatings typically qualify for programs like Energy Star and LEED and can save a homeowner an average of around 20-30% on energy costs.

Like all paint systems, SMP is permeable and should always be used in conjunction with a quality substrate like AZ50 Galvalume®. High quality resin + poor quality substrate = premature material failure. Quality must be paired with quality.

The battle between SMP and poly coatings typically takes place in exposed fastened panel systems like Tuff-Rib. Exposed fastened panel systems already represent a cost savings when compared to premium metal roofing systems like standing seam or metal shingles. For this reason some buyers can be tempted to chase the bottom dollar and settle for a poly coating on their roof in exchange for the lowest initial investment. And while the savings up front may seem like an okay deal (we're talking about a few hundred dollars compared to SMP systems on most roofs) the not so long term costs over the next five years will prove how bad of a deal it really is for the homeowner.

In conclusion, poly paint systems sold as a coating for residential metal roofing degrade the integrity and reputation of the metal roofing industry as a whole in the eyes of consumers who aren't aware of the differences in paint systems. An unaware person driving by a neighbor's home that features an unsightly fading, chalky metal roof may draw un unfair conclusion about all metal roofing systems in general and thus never consider one for their own use. This dilutes the market for everyone in our industry. It's important for manufacturers and suppliers to protect both their consumers and the metal roofing industry's health when making decisions about what products and coatings they offer by choosing to offer steel, substrates, and coatings that are truly designed for residential use.

Happy roofing!

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