Anti-corrosion coating is one of the most important protective measures for structural steel. Proper surface preparation, coating selection, application procedures, environmental control, and final inspection directly affect the durability and service life of a steel structure.

Before starting coating work, two basic requirements should always be followed:

• Anti-corrosion coatings should be applied only after the steel components have passed installation and quality inspection.

• All coating materials should be supplied with valid product certificates and applicable test reports. The brand, product type, and specification delivered to the site should be consistent with the approved documentation. Unqualified coating materials must not be used.

Important inspection note:
Anti-corrosion coating is generally treated as concealed work once it is covered by fireproof coating, cladding, insulation, or other subsequent construction layers. Therefore, coating inspection and acceptance should be completed and documented before the next covering process begins.

1. Surface Preparation — The Most Important Step

The performance of an anti-corrosion coating system depends heavily on the quality of surface preparation.

Even a high-performance coating may fail prematurely if rust, oil, moisture, welding slag, dust, or other contaminants remain on the steel surface.

1.1 Rust Removal Requirements

Steel surfaces should be properly cleaned and prepared before coating.

For many newly fabricated structural steel projects, abrasive blasting or shot blasting to Sa 2.5 is commonly specified.

At this preparation level, the steel surface should be substantially free from visible:

• Oil and grease

• Dirt and contaminants

• Mill scale

• Rust

• Previous coatings

Only very slight discoloration or traces may remain.

The prepared surface can be visually compared with the reference photographs specified in GB/T 8923.1 or other project-specified surface preparation standards.

1.2 How Soon Should Primer Be Applied After Blasting?

Freshly blasted steel can begin to develop flash rust quickly when exposed to air, especially under humid conditions.

For this reason, primer should be applied as soon as possible after surface preparation.

Typical site practice may include:

• Under normal conditions: primer applied within approximately 4 hours

• Under dry, favorable conditions: exposure should generally not exceed 12 hours

• For thermal metal spraying systems: approximately 12 hours in dry weather, with a much shorter exposure period under humid or rainy conditions

Actual allowable exposure time should follow the coating manufacturer's technical data sheet, project specification, and environmental conditions.

1.3 Final Cleaning Before Coating

Before applying primer, the surface should also be cleaned of:

• Welding slag

• Weld spatter

• Dust

• Grease and oil

• Moisture

• Burrs and sharp irregularities

Oil-contaminated surfaces should first be cleaned with a suitable degreasing agent. The surface must be dry before abrasive blasting or coating work begins.

2. Environmental Conditions for Coating Application

Correct environmental conditions are essential for coating adhesion and curing.

A useful reference for site control is the principle of temperature, humidity, and dew point management.

Temperature

A commonly controlled application temperature range is approximately:

5°C to 38°C

At excessively low temperatures, curing may become too slow or incomplete.

At excessively high temperatures, especially when the steel surface becomes very hot, the coating may dry too rapidly and develop defects such as bubbling, pinholes, or poor film formation.

Relative Humidity

Relative humidity should generally be controlled below:

85%

High humidity increases the risk of condensation and can reduce coating adhesion.

Dew Point

The steel surface temperature should normally remain at least:

3°C above the dew point

This is critical for preventing invisible condensation from forming on the substrate.

Weather Restrictions

Outdoor coating work should normally be suspended during:

• Rain

• Snow

• Fog

• Strong winds

• Heavy condensation

• Extremely intense sunlight when surface temperature is too high

Spray coating is generally not recommended under strong wind conditions.

Freshly applied coatings should also be protected from rain during the early curing period. As a general site precaution, exposure to rain within the first several hours after coating should be avoided unless otherwise permitted by the coating manufacturer.

3. Coating Preparation and Mixing

For two-component coatings, the base material and curing agent must be mixed according to the manufacturer's specified ratio.

The coating should be:

• Thoroughly mixed

• Prepared in suitable quantities

• Used within the specified pot life

• Applied according to the manufacturer's technical data sheet

Do not add excessive thinner simply to improve workability.

Coating products from different manufacturers should not be mixed unless compatibility has been specifically confirmed.

4. Coating Application

4.1 Application Methods

For large steel surfaces, airless spray application is widely used because it provides high efficiency and relatively uniform coating thickness.

For difficult or detailed areas, brush application is often used, including:

• Corners

• Bolted connections

• Welds

• Edges

• Small connection plates

• Complex joints

Stripe coating may also be applied to welds, edges, bolt heads, and other areas that are difficult to cover adequately by spray application alone.

5. Typical Anti-Corrosion Coating System

A common structural steel coating system consists of:

Primer → Intermediate Coat → Topcoat

The exact coating type, dry film thickness, number of coats, and total system thickness should always follow the project specification and coating manufacturer's requirements.

5.1 Primer

Primer should be applied as soon as possible after surface preparation has passed inspection.

A typical specification may require:

• 1–2 coats

• Approximate dry film thickness: 60–80 μm

The primer should form a continuous and uniform film without:

• Missed areas

• Runs

• Sagging

• Exposed substrate

5.2 Intermediate Coat

After the primer has reached the required drying or curing condition, the intermediate coat may be applied.

A commonly used option for industrial steel structures is an epoxy micaceous iron oxide intermediate coating.

Typical dry film thickness may be:

80–100 μm

The intermediate coating improves barrier protection, system thickness, and adhesion between coating layers.

5.3 Topcoat

The topcoat should be applied after the intermediate coating has sufficiently cured.

A typical system may use two topcoats with a combined dry film thickness of approximately:

60–80 μm

The finished surface should have:

• Uniform color

• Consistent appearance

• No sagging

• No pinholes

• No wrinkling

• No missed coating areas

Recoating Interval

Each coating product has both minimum and maximum recoating intervals.

Applying the next coat too early may cause solvent entrapment or poor curing.

Waiting too long may reduce intercoat adhesion.

If the maximum recoating interval has been exceeded, the surface should be treated according to the coating manufacturer's recommendations before applying the next coat.

6. Areas That Should Not Be Coated Before Site Welding

Areas adjacent to field welds may need to remain uncoated before welding.

The required uncoated width should follow the project drawings, welding procedure, and specification.

After welding is completed and the weld has passed inspection, the affected area should be:

1. Cleaned

2. Rust removed

3. Surface-prepared

4. Recoated using the specified original coating system

7. Dry Film Thickness Requirements

Coating thickness is a major factor affecting corrosion resistance.

If the dry film thickness is significantly below the specified value, the expected protection period may be reduced.

Where project drawings or coating specifications define the required thickness, those requirements should take priority.

For some projects without a separate design requirement, typical reference values may include:

• Outdoor steel structures: total dry film thickness of approximately 150 μm or more

• Indoor steel structures: total dry film thickness of approximately 125 μm or more

A negative tolerance of approximately 25 μm may be permitted under certain specifications.

However, the final acceptance criteria should always follow the applicable project standard and coating specification.

Dry Film Thickness Inspection

A calibrated dry film thickness gauge should be used.

A typical inspection approach may include:

• Measuring multiple locations on each selected member

• Taking several readings at each location

• Calculating the average reading

• Sampling a defined percentage of structural members

For example, one project specification may require five inspection locations per component, with three closely spaced readings taken at each location.

8. Repair Coating After Transportation and Installation

Steel components can be damaged during:

• Loading

• Transportation

• Unloading

• Lifting

• Assembly

• Site welding

Areas with damaged coating must be repaired.

A typical repair procedure includes:

1. Remove rust and damaged coating

2. Clean the exposed steel surface

3. Restore the primer

4. Apply the intermediate coat

5. Restore the topcoat

The repaired area should achieve the required coating system and dry film thickness.

9. Protection of Concealed and Difficult-to-Reach Areas

Special attention should be given to areas that will become inaccessible after installation.

Typical locations include:

• Inside sleeves

• Above suspended ceilings

• Behind cladding panels

• Enclosed connection areas

• Narrow gaps

• Internal or partially enclosed steel surfaces

Coating should remain continuous throughout these areas.

Installation sequencing should be coordinated carefully to prevent structural components, panels, or other building elements from blocking access before coating work is completed.

10. Quality Inspection

The completed anti-corrosion coating system should be inspected for appearance, thickness, adhesion, and repair quality.

Inspection Item	Inspection Method	Typical Acceptance Requirement
Surface Appearance	Visual inspection	Uniform coating without obvious wrinkles, sagging, pinholes, bubbles, or missed areas
Dry Film Thickness	Coating thickness gauge	Meets project-specified thickness and applicable tolerances
Adhesion	Cross-cut or pull-off test	Meets the applicable project or coating system requirement
Repair Areas	Visual and thickness inspection	Complete coating restoration with no exposed steel
Identification Marks	Visual inspection	Required marks, numbers, and identification remain clear

Inspection methods and acceptance criteria should follow the project specification, applicable standards, and coating manufacturer's recommendations.

11. Common Coating Problems and Corrective Actions

Problem	Possible Cause	Corrective Action
Peeling or delamination	Poor surface preparation, excessive humidity, incorrect recoating interval	Remove defective coating, reprepare the surface, and reapply the specified system
Insufficient film thickness	Too few coats or excessive thinning	Apply additional coating to achieve the required thickness
Blistering	High temperature, excessive humidity, or moisture on the substrate	Remove defective areas and recoat under suitable environmental conditions
Pinholes	Incorrect spray application or trapped air/solvent	Repair affected areas according to coating procedure
Rust staining	Incomplete surface preparation or coating damage	Remove corrosion, prepare surface, and restore coating system
Poor adhesion	Surface contamination or exceeded recoating interval	Remove loose coating, prepare the surface properly, and recoat

12. Safety Precautions During Coating Work

Anti-corrosion coating operations involve working at height, flammable materials, solvents, and spray equipment. Proper safety procedures are essential.

Personal Protection

Workers should use suitable PPE, including:

• Safety helmets

• Safety harnesses for work at height

• Protective goggles

• Protective gloves

• Respiratory protection

Spray coating personnel should use suitable respirators according to the coating material and site safety requirements.

Fire Prevention

Paints, curing agents, and thinners may be flammable.

Therefore:

• Smoking should be prohibited in coating areas

• Open flames should be kept away

• Paint and solvents should be stored correctly

• Welding and coating operations should not be performed simultaneously in the same hazardous area

If hot work is required, the coating condition and surrounding fire risk must be assessed and appropriate fire protection measures implemented.

Ventilation

Good ventilation should be maintained during coating work.

For enclosed or confined spaces, mechanical ventilation and appropriate confined-space safety procedures should be provided.

Waste Management

Unused coatings, solvents, contaminated materials, and cleaning waste should be collected and disposed of according to applicable environmental requirements.

They should never be discharged directly onto the ground or into drainage systems.

Equipment Maintenance

Spray equipment should be cleaned after use to prevent blockage and ensure stable spray performance.

Conclusion

A reliable anti-corrosion coating system for structural steel is not achieved simply by applying several layers of paint.

Successful corrosion protection depends on the complete process:

Surface Preparation → Environmental Control → Correct Coating Mixing → Proper Application → Film Thickness Control → Inspection → Repair and Maintenance

Among these steps, surface preparation and dry film thickness control are especially important.

For industrial plants, steel warehouses, workshops, logistics centers, commercial buildings, and other steel structure projects, the coating system should be selected according to the operating environment, corrosion category, expected service life, and project specifications.

At ZSJH STEEL, corrosion protection can be integrated into the steel structure fabrication and quality-control process, including surface preparation, coating application, inspection, packaging, and export preparation to meet different project requirements.