How Do You Clean Corten Metal Siding?

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Weathering steel claddings have become a popular choice in contemporary architecture and garden design, prized for their distinctive natural patina, exceptional durability, and low-maintenance properties. The dense rust layer (patina) that forms on its surface serves not only as a visual focal point but also as a ‘natural armour’ against wind and weather erosion.
However, cleaning corten steel siding differs from conventional metal facades – the core principle is not ‘thoroughly removing dirt’, but eliminating harmful impurities while preserving the protective rust layer.
This article shares scientific cleaning methods, prohibited practices, and maintenance techniques to help you keep your rusted steel cladding aesthetically pleasing and extend its lifespan beyond 50 years.

The Core Logic of Corten Metal Siding Cleaning

Corten Steel Cladding's Distinctive Nature

The fundamental difference between corten steel and ordinary steel lies in its stable rust patina. This layer forms through reactions between alloy elements like copper, chromium, and nickel with air and moisture. Dense and highly adhesive, it acts as a barrier against external corrosion, eliminating the need for additional painting or sealing.
The core principle of cleaning is: under no circumstances should this protective rust layer be compromised. Traditional metal cleaning methods involving abrasive polishing or strong acid/alkaline cleaners cause the rust layer to peel off, exposing the underlying steel to corrosive environments and consequently shortening its lifespan.

Why Is Cleaning Corten Steel Siding Necessary?

Although corrugated corten steel siding resists soiling, prolonged outdoor exposure leads to the accumulation of various impurities. If not promptly removed, these can compromise the stability of the rust layer:

Accumulation of surface dust, pollen, fallen leaves, etc., obstructs the rust layer's contact with air, causing uneven rust colouration in localised areas;
Salt spray in coastal regions and chemical pollutants in industrial environments accelerate localised corrosion;
Persistent stains like bird droppings and oil residues, if left adhered for extended periods, can penetrate the rust layer and compromise its integrity.

The purpose of cleaning is to remove these ‘harmful impurities’ without altering the rust layer itself, allowing the natural weathering process of corten steel cladding to proceed normally.

Classification of Common Contaminants

Mild impurities: dust, pollen, cobwebs, leaf debris;
Environmental pollutants: coastal salt spray, industrial dust, road dust;
Persistent stains: bird droppings, insect residues, rainwater marks, mineral deposits;
Harmful stains: oil residues, chemical agent residues (e.g., herbicides, fertilisers).

Preparatory Work Before Cleaning

Conduct a Comprehensive Inspection

Prior to cleaning, carefully observe the condition of the corten steel exterior walls to avoid indiscriminate actions:

Rust layer assessment: During the initial usage period (within 1 year), where the rust layer is not fully stabilised, handle with extreme care to prevent flaking. Where the rust layer is uniformly dense (2-3 years or older), cleaning pressure may be moderately increased;
Damage inspection: Check for impact-induced rust flaking, panel deformation, or water leakage traces at joints. Prioritise protection of such areas and address promptly post-cleaning;
Mark stains: Distinguish between ordinary dust and stubborn stains to plan cleaning sequences, preventing redundant work.

Essential Tools and Consumables

(1) Tool List

Basic tools: Garden hose with adjustable nozzle, soft-bristle brush (boar bristle or nylon), long-handled telescopic pole (for cleaning high areas), cleaning bucket, soft cloth;
Supplementary tools: Ladder (non-slip, stable, height suitable for external wall cleaning), gloves, safety goggles (to prevent dirty water splashing into eyes);
Prohibited Tools: The use of abrasive tools such as wire brushes, sandpaper, or scouring pads is strictly forbidden, as is high-pressure water jets (except under special circumstances).

(2) Cleaning Solutions

Primary Cleaning Solution: Clean water (preferably rainwater or tap water left to stand for 24 hours to reduce chlorine effects);
Mild soiling: Neutral washing-up liquid or laundry detergent (1/3 cup diluted in 6 litres of water; avoid excessive concentration);
Specialised stains: White vinegar (50% vinegar + 50% water, effective against mould and mineral deposits);
Prohibited Solutions: Never use bleach, ammonia, strong acid/alkaline cleaners, or abrasive scouring pastes.

3. Safety Preparations

Personal Protection: Wear rubber gloves, slip-resistant footwear, and safety goggles to avoid direct contact with cleaning agents or sharp edges on metal surfaces;
Environmental Protection: Cover plants, paving tiles, or wooden decking beneath exterior walls with plastic sheeting prior to cleaning to prevent contamination from debris or minor rust runoff;
Height Safety: When cleaning multi-storey building exteriors, use stable ladders or scaffolding. Never operate from unstable structures; engage professionals where necessary.

Step-by-Step Cleaning Protocol: Gentle Degreasing to Preserve Rust Layers

Step 1: Dry Cleaning to Remove Loose Debris

The initial cleaning step is ‘dry treatment’ to prevent debris from scratching the rust layer during wet wiping:

Using a soft-bristled brush or long-handled broom, gently sweep from top to bottom starting at the upper wall section to remove surface dust, cobwebs, fallen leaves, and other loose debris;
Pay particular attention to panel joints, corners, and other areas prone to debris accumulation to prevent particles from penetrating crevices during subsequent wet cleaning;
For tightly adhered dry debris (such as caked dust), avoid vigorous scraping. Instead, gently tap with a brush to loosen particles before removal.

Step 2: Pre-rinse to soften stubborn debris

Following dry cleaning, perform a pre-rinse using a garden hose:

Adjust the hose nozzle to a ‘shower spray’ setting (avoid high-pressure direct jets), rinsing from top to bottom to dislodge remaining loose debris;
For areas with stubborn stains, allow the water to gently wash over the surface for 1-2 minutes to soften the dirt, preparing it for subsequent cleaning;
Maintain a distance of over 30cm between the hose and the wall during rinsing to prevent excessive water pressure from impacting rust layers.

Step 3: Gentle Wet Cleaning to Remove Stubborn Stains

Following the pre-rinse, perform localised cleaning on residual stains:

Prepare a neutral cleaning solution (1/3 cup neutral detergent to 6 litres of water) and pour into a cleaning bucket;
Dip a soft-bristled brush into a small amount of solution and gently scrub the stained area using a ‘spot-brushing’ technique, avoiding vigorous rubbing over large areas;
Clean in sections from top to bottom, ensuring each area does not exceed 1 square metre to prevent solution residue drying on surfaces;
For details like joints and screws, use a soft cloth dipped in solution to wipe gently; avoid vigorous scrubbing with a brush.

Step 4 : Targeted Treatment for Specific Stains

For stubborn stains, select a gentle method appropriate to the stain type to avoid damaging the rust layer:

Mould / mildew: Apply a solution of 50% white vinegar and 50% water to a soft cloth, gently wipe the affected area, leave for 5 minutes, then rinse with clean water. The mild acidity of white vinegar breaks down mould without damaging the rust layer;
Bird droppings / Insect stains: Gently scrape off dried surface residue with a wire brush. Then, lightly press a cloth dampened with a neutral solution onto the stain. Once softened, wipe away without vigorous rubbing;
Mineral deposits (hard water marks): Gently wipe with a soft cloth dipped in a small amount of white vinegar. Repeat 2-3 times, then rinse with clean water. Avoid leaving vinegar on surfaces for extended periods.
Grease/oil stains: Apply a small amount of neutral degreaser directly to the affected area only (test on an inconspicuous wall section first to confirm no damage). Leave for 3 minutes, then rinse thoroughly with clean water, preventing the degreaser from spreading.

Step 5: Thorough Rinsing To Prevent Residue

Immediately rinse the wall with clean water to remove any residual cleaning solution:

Follow the same ‘top-down’ sequence, using a shower-like spray to thoroughly rinse every cleaned area, ensuring no foam residue remains;
Pay particular attention to joints, corners, and other areas prone to solution retention. Residue left here may form visible marks upon drying, affecting aesthetics;
After rinsing, inspect the wall. Should any uncleaned stains remain, repeat Step 3. Do not hastily resort to stronger cleaning methods.

Step 6: Post-Drying Inspection

Allow the wall to dry naturally (avoid rapid drying under direct sunlight). Conduct a final inspection once dry:

Examine the rust layer for integrity, checking for any flaking or discolouration caused by cleaning;
Check for any missed stains; if present, perform spot cleaning;
Confirm no cleaning solution residue remains on the wall surface. Minor water marks require no treatment and will be naturally washed away by subsequent rainfall.

Cleaning Techniques for Special Scenarios

1. Areas with Limited Rainfall Exposure (e.g., under awnings, beneath eaves)

These areas cannot be naturally cleaned by rain, making them prone to debris accumulation. Increase cleaning frequency (every 3-4 months):

Extend the pre-rinse duration during cleaning to allow water to thoroughly soften long-accumulated dust;
For tightly adhered debris, gently brush repeatedly with a soft-bristled brush, avoiding excessive force;
After cleaning, install drainage channels at the base of walls to channel subsequent light rainfall, reducing re-accumulation of debris.

2. Coastal Areas (High Salt-Spray Environments)

Salt spray in coastal regions accelerates corrosion of corten steel, necessitating focused cleaning:

Cleaning frequency: Clean every 2-3 months, increasing during rainy seasons as appropriate;
Cleaning focus: Repeatedly rinse the panel surfaces and joints with clean water to remove salt spray residue; no detergents required.
Additional protection: After cleaning, inspect the sealing condition at joints. Replace any aged sealant promptly to prevent salt spray penetration.

3. Industrial Areas (High Chemical Contamination)

Dust and chemical gases in industrial environments may cause discolouration of rust layers. When cleaning, note the following:

Prioritise rinsing with clean water to prevent detergent reactions with chemical contaminants;
Should abnormal discolouration (e.g., blackening or greening) appear on walls, refrain from self-treatment. First rinse with clean water and observe if the discolouration reverses. If persistent, contact professionals for assessment;
Increase cleaning frequency to every three months to promptly remove contaminants.

Cleaning Taboos: Absolutely Avoid These Practices

1. Prohibition on Harsh Chemicals

Under no circumstances should strong acids (such as hydrochloric acid or oxalic acid), strong alkalis (such as caustic soda), bleach, ammonia solutions, or similar cleaning agents be used. These substances directly corrode and dissolve the protective rust layer, exposing the underlying steel and accelerating corrosion.

2. Exercise Caution with Pressure Washers

The impact force of pressure washers can disrupt dense rust layers and even force moisture into the panel interior, causing leaks at joints:

If absolutely necessary, set pressure below 1000 PSI (lowest setting), maintain a distance of 3-4 metres, use a spray pattern, and only for rinsing large areas of dust. Never direct at stains or joints.
Avoid use whenever possible; a soft-bristle brush with clean water fully suffices for cleaning.

3. Prohibition of Abrasive Tools

Wire brushes, sandpaper, abrasive pads, stiff metal brushes, and similar tools will directly scrape away the rust layer, causing permanent damage. Even if a new rust layer subsequently forms, it will be uneven and non-compact.

4. Other Prohibited Practices

Do not clean in direct sunlight or high-temperature environments, as cleaning solutions will dry rapidly, leaving residue;
Do not allow cleaning solution to remain on surfaces for over 10 minutes; rinse promptly;
Avoid cleaning with automatic car wash brushes or hard plastic scrapers to prevent scratching the rust layer;
Do not seal or apply coatings after cleaning, as this impedes natural weathering and disrupts corten steel's self-protective mechanism.

Routine Maintenance: Key to Reducing Cleaning Frequency

1. Regular Visual Inspections

Conduct a simple monthly inspection, focusing on:

Significant accumulation of debris or stubborn stains (e.g., bird droppings, grease) on panel surfaces. Wipe promptly with a soft cloth to prevent long-term adhesion;
Leaks or ageing sealant at joints. Repair immediately to prevent moisture ingress;
Abnormal flaking or discolouration of the rust layer. Investigate causes (e.g., chemical exposure) and address promptly if detected.

2. Post-Rain Treatment

Following heavy rainfall, gently rinse walls using a garden hose with a spray nozzle. Combining rainwater with manual rinsing reduces residue buildup and extends cleaning intervals.

3. Seasonal Maintenance

Spring: Conduct a thorough clean after winter to remove accumulated debris and prepare for year-round use.
Autumn: Clean before winter sets in to remove fallen leaves and dust, preventing winter rain or snow from freezing debris onto the surface.
Winter: Reduce cleaning frequency. Should snow accumulate on walls, allow it to melt naturally before gently rinsing with clean water. Avoid vigorous cleaning in cold temperatures, which may cause the rust layer to become brittle.

4. Protecting the Surrounding Environment

During the first year after corten steel installation, the rust layer remains unstable and may cause rust water runoff. Lay gravel or waterproof sheeting beneath walls to prevent staining of paving stones or concrete surfaces.
Avoid applying herbicides, fertilisers, or other chemical agents near the wall surface. Should accidental contact occur, rinse immediately with clean water.
Regularly clear fallen leaves and weeds from the wall perimeter to minimise sources of debris.

FAQ about Corten Metal Siding

Q1: How Often Should Corten Steel Exterior Walls Be Cleaned?

Under normal conditions, cleaning twice yearly is recommended (spring and autumn). In coastal, industrial areas, or dusty environments, cleaning should occur every 2-3 months. In sheltered areas with little rainfall, cleaning every 3-4 months is sufficient.

Q2: Will Cleaning Remove The Rust Layer?

Proper cleaning will not dislodge a stable rust layer. Gentle cleaning merely removes surface impurities without compromising the dense rust layer structure. If rust flaking occurs post-cleaning, this likely stems from improper tools or cleaning agents, or an unstable rust layer (typically during initial use).

Q3: Can Tap Water Be Used Directly For Cleaning?

Yes. Tap water is the primary cleaning agent for corten steel. Only when encountering stubborn stains should a small amount of neutral detergent be used. It is recommended to let tap water stand for 24 hours before use to allow chlorine to evaporate, minimising its slight impact on the rust layer.

Q4: Will Rainwater Wash Away The Rust Layer?

No. A stable rust layer (after 2-3 years of use) develops a dense texture with strong adhesion, making it impervious to rain. During the initial phase (within the first year), the rust layer is not fully stabilised, and some rust dust may be washed away by rain. This is normal and requires no concern, as a more stable rust layer will subsequently form.

Q5: Does It Require Sealing or Painting After Cleaning?

No. Sealing or painting would seal the corten steel surface, preventing contact with air and moisture. This disrupts the natural rust layer formation, leaving the underlying steel unprotected and accelerating corrosion. Corten steel's advantage lies in its ‘self-protection’ – no additional coatings are needed.

Conclusion: The Core Principle of Cleaning Is ‘Working With Nature’

Cleaning weathering steel roofing fundamentally involves ‘assisting its natural weathering process’ rather than ‘altering its essence’. By adhering to the principles of ‘gentle treatment, preserving the rust layer, and avoiding chemical intervention’, and conducting basic cleaning twice annually, its aesthetic appeal can be maintained while extending its service life beyond 50 years.

Compared to traditional metal facades requiring frequent repainting and sanding, Corten steel's low-maintenance properties save significant time and costs. Proper cleaning and upkeep not only preserve its distinctive rust patina but also ensure this eco-friendly, durable building material becomes a valuable asset for long-term property value retention.

Should you have further queries regarding corten cladding cleaning, please contact us for bespoke advice, ensuring your corten steel exterior remains the architectural “face” of your building.