MPI Panel Finish 101

A few months ago, I was driving north on I-35 through Kansas at sunrise. It was a picturesque rhythm of farmland occasionally syncopated by an old barn near the highway. These old barns had outlived their usefulness, and had degenerated into roadside billboards painted on their corrugated metal roofs. Even the advertising was a thing of history now. Rusting and fragile, the barns were ghosts of their former purpose. I couldn’t help thinking of how different metal panels are today, and how much longer they will last.

Today’s metal roofing and metal buildings may seem to be closely related to those old metal barns strewn along rural highways, but nothing could be further from the truth. Over the past decade, metal corrosion technology and paint chemistries have catapulted metal panels into a truly archival construction material of choice. To understand the quality, durability and beauty of current metal products, we’ll take a look at the current paint coating technology, and the coil coating process that makes these new panels so amazing.

At the most fundamental level, great metal panels begin with corrosion resistance. Surprisingly, this is NOT the primary function of the paint; rather, corrosion prevention begins as a pre-treatment to the metal itself. Panels are made from metal coil stock, and must be hot-dipped in a galvanize or galvalume process, cleaned, etched and chemically treated with either zinc phosphate, chrome-based or dried-in-place compounds before painting. These treatments chemically alter the surface of the metal creating a corrosion-resistant metal with a much-improved surface for paint adhesion. The most favored treatment for coiled steel today is galvalume for it has been tested to outperform galvanized steel in many weather environments. The challenge is that the very treatments that make panels corrosion-resistant, also make it the most difficult material to paint. Special surface preparation and uniquely formulated primers are necessary to achieve excellent paint adhesion.

Although the processes vary with different paint formulations, pre-treated metal usually receives a primer and topcoat and even a final UV coating to complete the finished, pre-painted coil. Let’s explore this process in more detail.


Once the metal coil has gone through the pre-treatment step, a primer coat is applied. In the past, epoxy or epoxy-esters were commonly used. Today polyester, polyurethane or acrylic water-based primers are preferred for their flexibility and resistance to ultraviolet light. Some coil coaters, use unique, chemically-active primers to improve the bond between steel and coatings. Sherwin-Williams Corporation uses their patented chromate-activated primer to chemically alter the surface of the metal for better adhesion properties, and enhance corrosion resistance as well. Although only 0.25 mils thick, the primer is one of the most critical elements to panel longevity.

The topcoat is the finish that gives the metal its color and sheen, and it’s most important function is to protect against weathering and UV exposure. There are many different formulations for this topcoat, and these choices are a balance between price and performance. There are economy formulations best suited for interior or low exposure applications, to advanced coatings proven to handle the harshest of environments. In general, coatings are primarily made up of a pigment, resin and solvent, although we differentiate them by their resin type. Polyesters are generally considered an economy finish, primarily used for soffit, signage, industrial or agricultural applications and have a relatively low exposure and UV resistance. Newer polyester formulations, especially those blended with ceramic pigments offer outstanding performance improvements, although still not at top performance levels. Vinyl-plastisol coatings offer a unique thick film surface and handle slightly more UV and environmental exposure than polyesters. Silicone-modified Polyester resins offer significantly more UV resistance (the higher the silicone content the better the finish performance), and better suited to handle environmental exposure including humidity, salt air, acid rain, air pollution or agricultural hazards. The current benchmark standard of the industry is a 70% PVDF, or fluoropolymer resin, such as Kynar 500 or Hylar 5000. PVDF is usually specified as a two-coat system, although special paint features may cause the finish process to be up to four coats. This finish has outstanding weathering properties and handles high levels of UV, humidity, salt air and more. Metallic finishes like the Fluropan Classic II (Sherwin-Williams) contain metal flakes or oxides to create the metallic depth, and therefore require an additional clear PVDF topcoat to protect against oxidation and ultraviolet light. Metallic finishes are created using either aluminum flake or mica powder. The mica powder finish tends to be more uniform in appearance. Unique to metallic panels, care must be taken when installing the panels because the finish is directional; panels will appear to have a different depth or shading when viewed from opposite directions. For this reason manufacturers mark the back side of these panels with arrows, to ensure all panels are installed in the same direction. This directional characteristic of metallic panels is called “color flop” and is caused by the method of continuous painting which lays down all the metallic material at the same angle. If a panel is inadvertently “flopped” during installation, the metallic grain will be going a different direction from the others, causing the panel to look significantly different from the rest. New two-toned texture paint and sand-coat finishes round out the wide range of product looks available.

Topcoat finishes are created not only for appearance, but to protect against exposure conditions of sunlight, heat and moisture. Inevitably, weathering will degrade all paint over time. The better the weather resistance of the finish, the longer the life the panel will have. Often, we ask, “How long will the finish last?” when we really mean “How long will the finish retain its original color and gloss?” While the panels may have a long life beyond its original “like new” appearance, the finish performance is based on pigment stability and resin durability. Ultraviolet light, the real foe of panel finish, chemically breaks down the coating. UV exposure will cause fading, which occurs by the gradual breakdown of the pigment by the sun. Depending on finish and color, fading might appear as a bleaching away of the original color, or may exhibit a lateral shift from one color toward another. Organic pigments may be more susceptible to fade than ceramic pigments. Coil coaters may indicate whether the finish pigment is organic, inorganic or ceramic, and should specify if the pigment is a blend of the pigment types. Chalking, another common finish failure, is caused by the breakdown of carbon bonds in the finish and exhibits a white, powdery and irregular substance on the panels.

All certified paint finishes have been real-time or accelerated-exposure tested and warranted for a specific period of time. Most testing and certification is done today base on accelerated-exposure testing; this type of testing is far from accurate, since the rate of paint failure is not linear and the exposure to the elements will vary. Additionally, these warranties need to be studied carefully, for they differ widely. The three key warranted characteristics are film integrity (the coating’s long-term ability to adhere to the metal), weathering (gloss, chalk & fade in specific Hunter units), and corrosion resistance. Depending on your location and proximity to environmental hazards, corrosion warranties may not be issued. If required, check for products that carry a corrosion warranty in a seacoast or high-chemical environment.

One warranty may offer a comprehensive manufacturing defect coverage, while another may merely guarantee that it is “paint”. Therefore, one must look beyond the 25-year or 40-year bold type to see what the warranty actually covers. Conservative warranties such as Berridge’s 25-year PVDF, actually warrant against a measurable amount of appearance chalk or fade, even though their protective qualities will last much longer.