Weathering Steel Bleeding Prevention Guide
This guide will show you how to prevent bleeding of weathering steel onto adjacent surfaces. It’ll also share mitigation, common handling mistakes, and how to maintain the material.
DMD Editor | Dissimilar Metal Design
Article Updated June 29, 2026
The Ultimate Guide to Weathering Steel
Our team at Dissimilar has been working with weathering steel for over three decades. As a result, we know a lot about how to develop this beautiful material. We know its pitfalls, how to avoid them, and how to give it an amazing patina that will last without bleeding.
Here is an index of this guide with some quick links to jump to sections.
Weathering Steel Basics
Weathering steel is a material known by many names:
- The genericized old registered trademark COR-TEN Steel which was owned by US Steel.
- More casually as “corten steel”
- Occasionally as “Copper-bearing steel” (because of the copper alloying element it contains).
- By its alloy numbers, A588 and A606
While it has multiple monikers, weathering steel is singular in its distinctive rustic appearance and its corrosion resistance.
With that said, its biggest challenge is its potential for “bleeding.” If installed in a raw or insufficiently weathered state, rust particles wash off the surface and stain nearby structures.
In this article, we’ll look into the methods for preventing bleeding and mitigating and cleaning when it does. In addition, we’ll discuss the importance of preweathering and its underlying science.
Dissimilar Approach to Weathering Steel
At Dissimilar Metal Design, we’ve produced millions of square footage of preweathered corten steel.
Our approach is to use a proprietary patina that we developed, and to apply that to prefabricated parts. By doing this, minimal further alteration is required in the field and thus protects the material from new oxidation.
Regular Steel vs. Weathering Steel
Although indistinguishable to the naked eye, weathering steel is not the same as regular mild steel. When typical steel alloys rust, they will continue to do so, deteriorating until the material completely breaks down. Weathering steel contains copper and other alloying which change its molecular structure.
When weathering steel is properly patinated, it develops a protective oxide layer. This patina shields the steel from further corrosion, making it ideal for outdoor applications. However, achieving the stable patina requires proper fabrication detailing and surface preparation.
What happens if we don’t preweather?
We’ve established that weathering steel has great longevity if properly detailed and installed. It will not break down like regular mild steel.
However, in its raw state, or if incorrectly fabricated and installed, it will bleed (a lot).
This is because exposed raw weathering steel will oxidize and create rust particles, which bleed down as these particles get washed downward by rainwater.
In addition, it can take years or decades to naturally develop a stable patina in situ, if one develops at all. This means that if installed in a raw state or partially oxidized, it can bleed onto other surfaces for years. Not to mention, cleaning rust off of other materials can prove difficult.
A science-based approach to preweathering, like what we provide at Dissimilar Metal Design, can prevent bleeding from happening. This enables designers to specify weathering steel next to other materials without worrying about staining.
The Science of Weathering Steel
Understanding the science of how a patina forms on weathering steel can give you an edge in conversations with end users of natural metals. Its important to be able to determine whether or not the weathering steel patina is fully stabilized.
Below we’ve outlined some of the basics, and provided some weathering steel infographics that break down the steps:
Protective Oxide Layer Development
This graphic illustrates the electrochemical process of iron (Fe) rusting, which involves the formation of rust (iron oxides) when iron reacts with oxygen (O₂) in the presence of water (H₂O). This process is also known as the corrosion of iron.
They show how oxygen participates in the rusting process, with the aid of water, and highlight the necessity of an electron donor — in this case, iron, which loses electrons (oxidizes) elsewhere on the metal’s surface.
DMD’s processes are setup to ensure that these protective oxides are consistently developed and details are designed to ensure their longevity.
The above chart is a simplified representation of a more complex and dynamic process. Over time, the rusting process can result in the formation of different iron oxides and mixed oxides which may include lepidocrocite (γ-FeOOH), goethite (α-FeOOH), and others depending on the environmental conditions (see below).
These different types of oxides make a big difference in determining whether the metal has a protective coating.
Understanding when the patina is stabilized is crucial. There are several types of rust that form on the surface of weathering steel. It is important to determine which type of rust is appearing on the surface. This is because the type of rust formation determines the stability of the patina:
Protective Oxide Layer Structure
When rust first develops on weathering steel, a bright orange oxide called Lepidocrocite is formed and takes over the surface.
After sequential mechanical and chemical etching cycles, the rust layers begin to refine into Maghemite and Goethite.
Over time, these particles become much smaller than typical rust. This allows the particles to bond together. These finer, more tightly bound structures are called Nano-phase Goethite.
While nano-phase goethite can be part of mature rust layers, it’s not a guaranteed outcome of the natural weathering process.
The thickness of goethite on weathering steel can vary depending on the environmental conditions and the duration of exposure. Typically, the layer of goethite that forms on weathering steel as part of the rust layer can range from very thin films to tens of micrometers thick.
For a precise measurement or a specific case, it’s usually determined through a detailed analysis of samples using techniques like microscopy or other surface analysis methods. If you need the thickness for a particular type of weathering steel or under specific conditions, consulting technical data sheets from the manufacturer or conducting an experimental measurement would be necessary.
To recap, here are the basics of the weathering process:
- Initial Corrosion: When exposed to moisture and air, weathering steel begins to rust. The initial rust is porous and loosely adherent.
- Patina Formation: Over time, with accelerated oxidation cycles, the rust layer undergoes chemical changes. This results in the formation of a more adherent and less porous oxide layer.
- Stabilization: A stable patina forms, providing protection against further corrosion and reducing the risk of rust runoff.
Thus, the key to preventing bleeding lies in properly developing the protective rust layer, prior to delivery and installation.
Prevent Bleeding on Weathering Steel
We’ve established how weathering steel’s rust formation occurs. So how best to prevent this rust formation from running down the surface and onto adjacent surfaces?
The short answer: use the DMD preweathering process. You want to use it post-fabrication, and be sure that it is properly packaged, delivered, and installed without damaging or altering the metal system during installation.
DMD’s preweathering system is the best way to prevent bleeding on weathering steel, because it makes this process happen before your panels are installed. This is a controlled process that accelerates the formation of the patina. This occurs after the material has been fabricated into parts, but before the steel is installed in its final location.
Preweathering exposes the corten steel in a controlled environment. This typically occurs at the fabrication facility that is set up for wet/dry cycles. The material undergoes the initial corrosion and patina formation stages, resulting in a fully stabilized patina. Here’s how it works:
- Controlled Exposure: The steel is placed in an environment with controlled oxidizing cycles. This simulates natural weathering conditions but at an accelerated pace.
- Chemical Treatments: Chemical treatments are applied to the steel surface to promote faster rusting and patina formation. These treatments can include solutions of chlorides, sulfates, or other oxidizing agents.
- Monitoring and Testing: Throughout the preweathering process, the steel is monitored to ensure that the patina is forming correctly. Tests may include visual inspections, durability tests, and measurements of the oxide layer thickness.
Benefits of Preweathering
By forming a stable patina before installation, the risk of rust runoff and staining is significantly reduced. Preweathered steel has a time-earned appearance, enhancing the visual appeal of the structure from day one. Controlled preweathering ensures that the steel’s protective layer is predictable, enhancing its long-term durability.
With a properly preweathered panel, you can confidently install.
Designing to Mitigate Bleeding of Weathering Steel
In addition to preweathering, thoughtful design can play a critical role in mitigating the bleeding of weathering steel. By considering the pathways for runoff and minimizing the need for alterations to preweathered panels, designers can ensure that weathering steel maintains its intended appearance and structural integrity. This section explores strategies for effective design to prevent rust staining and maintain the aesthetic of weathering steel structures.
Control where Runoff Goes
- Sloped Surfaces: Ensure that surfaces are sloped to direct water away and prevent any standing water. This can be achieved by angling panels or incorporating slopes into the design of roofs and facades.
- Drip Edges: Install drip edges or reveals at the bottom of panels to channel water away from the structure. These prevent water from collecting on face of the building.
- Gutters and Downspouts: Use gutters and downspouts to collect and direct runoff to safe discharge areas. These elements should be designed to handle the volume of water expected and positioned to avoid staining visible surfaces.
Installing Raw Steel: Design for Runoff
If you’re not preweathering your panels, then you should plan for bleeding. For instance, you can design rock basins or natural trenches to collect runoff. If large amounts of runoff discolor the rock basin, you can simply rotate the gravel as part of your building’s maintenance plan.
Designing with Weathering Steel
Weathering steel, with its distinctive rusty appearance, brings a unique aesthetic appeal to architectural and structural projects. That said, designing and installing weathering steel systems requires care for the details.
We hope this article has helped arm you with some knowledge for how to effectively design and install custom fabricated and preweathered corten steel.
Our team at Dissimilar has been detailing and fabricating weathering steel systems for many years. Take a look at some of our highlights below, and reach out to us with any questions or projects that you’d like us to take a look at.
