Rust inhibitor primer on commercial metal roof

When installing a coating system on a rusty metal roof, we strongly recommend using a rust-inhibiting primer—In fact, we require it. While most commercial roofers know full well what rust can do to a metal roof, few understand exactly how it can negatively impact the performance of a metal roof coating system.

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Why Is Rust a Problem for Metal Roofs?

Before we get into why rust is bad for commercial metal roofs, let’s first get a basic understanding of what rust is exactly.

According to Wikipedia: rust is an iron oxide, usually a reddish-brown oxide, formed by the reaction of iron and oxygen in the presence of water or air moisture. To put it plainly, rust is essentially a chemical process that results in the disintegration of a material.

So, why is rust a problem for metal roofs? Over time, rust will cause the metal roof panels to weaken, thus compromising the structural integrity of the roofing assembly. That is why it can be unsafe to walk on a metal roof with significant amounts of rust, especially if holes or flaking are visible.

If not addressed early on, rust will eventually corrode the metal to the point where total replacement is the only viable option.

What Causes a Metal Roof to Rust?

In order for rust to form on a metal roof, three elements need to be present: metal, oxygen, and moisture.

Most of today’s metal roofs are manufactured with a protective galvalume coating, which consists of zinc, aluminum, and silicon. It’s similar to the “galvanizing” process because it acts as a “sacrificial” covering to protect metal roofing materials (primarily steel) from oxidation.

The galvalume coating will wear off over time due to exposure to the sun and other weather conditions, allowing the three rust-causing elements mentioned above to thrive.

What Is a Rust-Inhibiting Primer?

As the name suggests, a rust primer inhibits the spread of rust and corrosion on metal surfaces. They are designed to penetrate and bond to heavily rusted metal to provide a surface for which an elastomeric roof coating will adhere to. Most rust primers are­ red in color and have the same physical characteristics as thin oil-based paints.

The main function of a rust inhibitor primer is to eliminate oxygen and moisture, two of the three elements that cause rust. This is accomplished with zinc phosphate, which is the key ingredient in a rust-inhibitive primer. Zinc phosphate acts much like the galvalume coatings found in newer metal roofing materials—it serves as the “sacrificial” coating to protect bare metal surfaces.

When Is a Rust Primer Required?

While we can’t speak for all coating manufacturers, American WeatherStar metal roof restoration systems require a rust-inhibiting primer (Red Oxide Rust Prime 912) if any level of rust is present on the roof surface. We also require a primer if the metal has been previously coated or factory-painted to ensure proper adhesion of any subsequent coating applications.

The application guidelines for American WeatherStar metal roof restoration systems (Met-A-Gard, Met-A-Gard+, and Met-A-Sil) specify when and how to apply Red Oxide Rust Prime 912, as well as the required coverage rates. If there is any question as to whether a primer is needed, we strongly recommend performing an adhesion test to confirm suitability. Contact our Technical Services department if you wish to receive an adhesion test patch kit, or if you have any additional questions.

VIDEO: Click here to learn how to perform a roof coating adhesion test patch.

How to Apply a Rust-Inhibitive Primer

Rust covering primer is typically applied with an airless sprayer at a capacity of a 1/2 gallon per minute utilizing a .017 spray tip. If necessary, they can also be roll-applied or brush-applied to avoid overspray or in smaller roofing applications.

To ensure a successful application, it is critical to determine how much primer will be needed to prevent the rust from bleeding through. Rust bleed-through appears in the form of red specs showing through the top coat. This happens when the adequate amount primer is not applied.

The amount of coating (coverage rate) is based on the porosity of the rust. Think of rust as if it were made of microscopic peaks and valleys. The primer must be applied at a rate to not only immerse the “valleys,” but to eclipse the “peaks” as well. Otherwise, the bleed-through of rust is inevitable.

Contractors should always follow manufacturer specifications by referring to the product data sheets, safety data sheets, and application guidelines.

Rust Inhibitor vs. Rust Converter

A rust-converter primer is a strong phosphoric acid that converts iron oxide (flaky metal and low strength) to iron phosphate (very stable and has decent cohesive strength)—however, you are only treating the material that has already been compromised. It’s also worth mentioning that rust converters are made up of strong acids and can be difficult to work with.

A rust-inhibitor primer, on the other hand, is designed to provide a zinc phosphate source to eliminate the advancement of rust and corrosion and to prevent moisture from reaching rusted metal.


Addressing rust issues with a primer beforehand is paramount when installing an elastomeric coating system on a commercial metal roof. In the end, using a rust-inhibiting primer is a small price to pay when it comes to the performance and longevity of a metal roof restoration system.

If you would like to learn more about the various primers and cleaners that we offer to commercial roofing contractors, click here to locate and speak with an American WeatherStar field representative near you.