Rust Staining on Metal Cladding
Why rust staining appears on commercial cladding, what it indicates about the source corrosion, and how to address the cause — not just the visible symptom.
6 min read
- Rust staining is a symptom — the visible stain is run-off from active corrosion occurring above or behind it.
- Treating the stain without identifying and treating the corrosion source will result in recurrence.
- The most common sources are corroded fixing heads, cut edge corrosion, and through-panel corrosion.
- The corrosion source determines the treatment — ProFix for cut edges, fixing replacement, or structural repair as appropriate.
- Rust staining provides diagnostic information that guides the inspection — use it, do not simply clean it.
What Is Rust Staining?
Rust staining is brown or orange-brown discolouration on the face of cladding panels, caused by water washing over corroding steel and depositing iron oxide (rust) as it flows down the surface. The stain itself is cosmetic — it is iron oxide deposited on the coating surface, not corrosion of the panel face at the stain location. However, the stain's presence and pattern provide valuable diagnostic information: it identifies where active corrosion is occurring above the stain, and it indicates that corrosion has reached the stage where rust particles are being washed off in quantity.
Why Does It Happen?
Rust staining originates from a point source of active corrosion that is washed by rainwater flowing down the cladding surface. The common sources are:
- Corroded fixing heads: Self-drilling fixings with failed or absent neoprene washers, or fixings where the zinc or paint coating on the head has failed, corrode and produce rust run-off in characteristic vertical stain patterns running directly down from the fixing position.
- Cut edge corrosion: Active cut edge corrosion at eaves, ridge or side laps produces rust run-off that tracks down the elevation below the affected edge. The stain typically appears as a broad wash rather than a point-source drip.
- Surface coating failure with steel exposure: Where the original coating has failed to the point of exposing the steel substrate in localised areas, surface oxidation produces staining in the pattern of the exposed area.
- Through-panel corrosion: On older buildings, corrosion originating on the internal face of a cladding panel — typically from condensation exposure over many years — can corrode through the panel thickness and produce staining on the external face. This is less common but more structurally significant than the other sources.
How Is It Assessed?
Assessment maps the stain pattern and traces it to its source. Fixing-origin stains are typically narrow vertical streaks running from visible fixing positions. Cut edge origin stains are broader washes running from eaves or lap positions. Internal corrosion origin is identified by inspection of the internal cladding face and correlation with external stain patterns. Drone photography can map stain extent on large buildings, but close inspection from access equipment is required to assess the corrosion source.
When Refurbishment May Be Suitable
Once the corrosion source has been identified and treated, the stained areas can be incorporated into a wider coating programme. On a building where cut edge corrosion is the primary source, ProFix treatment at the affected edges followed by a full facade refurbishment with TopSpray 35 or SprayGrade addresses both the source corrosion and the cosmetic staining. Where fixing heads are the source, replacement of corroded fixings or application of fixing head sealant caps followed by the coating programme addresses both issues.
When Refurbishment May Not Be Suitable
Cleaning rust staining from the panel face without treating the source corrosion will result in staining recurrence within months. The stain is not the problem — it is the indicator of the problem. Surface cleaning alone is not a treatment.
Where staining indicates through-panel corrosion from the internal face, structural assessment is required before any external coating programme. External coating applied over a panel corroding from within will fail as internal corrosion continues to undermine the panel from the inside face. Panel replacement may be the required response in these cases.
Systems Commonly Considered
ProFix is the primary treatment system where cut edge corrosion is the staining source — it converts active rust at the edge, applies inhibitive primer, and seals the edge against further moisture ingress. Where surface coating failure is the source, preparation and top coat with SprayGrade or TopSpray 35 addresses both the source and the stained areas. Fixing head treatment (sealing caps, replacement or head application of a localised rust converter) is a separate scope item that precedes the main coating programme.
Rust staining is diagnostic. When we inspect a building with extensive rust staining, the stain patterns tell us more about the building's condition than any other single indicator available from the ground. A fixing-origin stain pattern running in rows tells us the fixing specification used, the age at which fixing protection began to fail, and (from the stain density and colour intensity) roughly how active the corrosion currently is. A broad eaves-to-mid-elevation wash tells us there is significant cut edge corrosion at roof level that has not been visible from ground level.
Our inspection process uses stain pattern analysis to prioritise the close-inspection areas before access equipment is deployed. On a large building, this significantly improves the efficiency of the condition survey and ensures the assessment time is spent where the most useful information will be found.
Common Mistakes
- Cleaning rust staining from the panel face without identifying and treating the source — recurrence is predictable.
- Not including fixing inspection in the scope when rust staining is present — fixing origin staining is common and easy to overlook.
- Interpreting through-panel staining as surface or fixing origin without internal inspection — the treatment options are different.
- Applying top coat over stained areas without preparation — iron oxide residue must be removed from the surface before coating or it will interfere with adhesion.
Frequently Asked Questions
Can rust staining be removed from cladding?
Yes. Iron oxide staining on the cladding face can be removed by pressure washing and, where necessary, application of a proprietary rust remover solution before the final wash. The stained area can then be incorporated into the coating programme. Surface cleaning without source treatment will result in recurrence.
Does rust staining mean my cladding needs replacing?
Not necessarily. Rust staining indicates active corrosion at a source location — but the extent and nature of that corrosion determines whether treatment or replacement is appropriate. Fixing-origin staining is typically resolved by fixing treatment and coating. Cut edge origin staining is typically resolved by ProFix treatment. Only through-panel structural corrosion may indicate replacement is required.
What causes rust staining in rows down the cladding?
Rows of rust staining aligned with fixing positions indicate fixing head corrosion. The neoprene washer under the fixing head has failed, or the zinc coating on the fixing head has been consumed, allowing the steel shank to corrode and wash rust down the panel below.
Is rust staining a structural concern?
The stain itself is not structural. The corrosion producing the stain may be. The assessment determines the structural significance of the source corrosion — most cases are maintenance issues rather than structural emergencies, but assessment is required to confirm this.
Related Guidance
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