A metal roof is a durable roof, maintaining its strength and integrity for decades. But it can always use some help. Most steel comes with a coating, called a substrate, over which a colored or metallic finish can be placed.
Galvanized steel is the most common coated steel in use. Paint and other finishes can be layered over the galvanized coating at the manufacturer, or they can be painted after the panels are formed. Paint is also used for maintenance and repair of the roof panel surface as it weathers.
Let’s find out the benefits and objectives of painting steel panels, what the paint is made of, and the types of paint available for your metal roof. We’ll talk a little about performance expectations, application, and warranties while we’re at it.
Why should your metal roof be painted?
While steel panels already have a protective coating, paint provides another layer of protection. The roof may need to have a different color than natural metal. Maybe your roof needs to match others nearby, or you want to save energy by installing a cool roof.
Paint enhances the aesthetics and performance of your roof panels while protecting the substrate and underlying metal. Galvanization resists rust by slowly melting away, sacrificing itself in preference to the steel underneath. Over time the protective coating can wear away. Paint layered over the galvanized coating delays this process by protecting the substrate.
What is paint made of?
Paint and finishes for metal panels have three components.
- Resin bonds to the steel substrate and forms the desired film.
- Pigment provides color and hides the primer and the substrate. Both resin and pigment are solids and require a carrier fluid.
- Solvent is added to create a liquid paint. Once the paint is applied, the solvent evaporates, leaving the pigmented resin attached to the metal.
Sometimes additives are also mixed into the paint to enhance performance. Some control the foam, flow, and leveling of the paint as it is applied while others modify the viscosity. Occasionally, a catalyst is added to accelerate the desired chemical reaction.
Paint is identified by the resin type, including acrylic, epoxy, fluorocarbon, polyester, and urethane.
Polyester resins are formulated to balance performance with economic criteria. The resins differ depending on the characteristics and performance required for a specific roof application.
The polyester resins are available as silicon polyesters and super polyester as well as silicon-modified polyester (SMP). One silicon polyester is called polyvinylidene fluoride or PVDF (which is much easier to say).
- PVDF can withstand extended exposure to moisture, extreme temperatures, UV radiation, oxygen, and atmospheric pollutants. It is frequently specified for metal roofs. PVDF is 70% polyester resin and 30% acrylic resin.
- Another silicon or super polyester is known as FEVE. FEVE resins are resistant to exterior elements and have low permeability to oxygen, water, and chloride - highly corrosive conditions. It offers a higher gloss and wider color range compared with PVDF resins.
- SMP, silicon-modified resin, is not as formulation-specific as acrylic and polyester resins. It provides good weather resistance and is more economical than FEVE and PVDF.
Pigments can be organic or inorganic. Organic pigments offer bright colors but are not as color stable as inorganic pigments. They tend to fade quicker.
Inorganic pigments, also known as ceramic, are complex metal oxides that provide earth tones and whites. They are more resistant to fade, but there are no bright reds or yellows.
Coating manufacturers reference ASTM specifications and test protocols for evaluating a long list of performance metrics.
- Specular gloss
- Color consistency
- Film hardness and thickness
- Abrasion resistance
- Chemical resistance
- Salt-spray and corrosion resistance
The primary conditions that most commonly degrade paint as sunlight, heat, and moisture. You shouldn’t be surprised to learn that a prominent paint testing ground is located in South Florida where there is plenty of all three. Less common issues are caused by airborne chemical pollutants of one sort or another and rain.
The adhesion parameters apply to a paint’s ability to resist cracking, blistering, and peeling. Color consistency measures the amount of fade a particular color and type of paint experiences over time.
Paint Application on steel roofing panels
The preferred method of applying a finish to metal is the coil-coat process, which ensures a uniform gloss, color, and thickness along the entire coil of steel. The rolls of metal are painted before they are formed into roof panels. Coil-coat painted metal can be cut, slit-formed, corrugated, profiled, or molded into almost any shape.
Factory coating may require touch-up painting and maintenance within the first five years. Every three to five years after that a new coat is advised. Any new paint should match the attributes of the original paint, and any repainting should be performed after repairs are complete.
The surface also should be power-washed to get rid of dirt and grime. Rust spots should be scrubbed with a wire brush and treated with a rust inhibitor.
A warranty generally covers film adhesion and the maximum levels of allowable chalk and fade within the warranty period. However, warranties do not usually include under-film corrosion, cracks, cut edges, or other conditions. Be sure to check the fine print.
Paint finishes for metal roofs are important elements of protection against extreme environments and day-to-day exposure to the weather. They come in a wide variety of colors and gloss. If you require something with extraordinary wear, FEVE or PVDF resin will perform better than other resins.
Maintain your roof with repairs, cleaning, and new paint every few years and your building will continue to look beautiful and protect the contents inside for years to come.