Steel loses strength rapidly when exposed to high temperatures. Applying a suitable fireproof coating can delay the temperature rise of structural steel, maintain its load-bearing capacity during a fire, and help the building achieve the required fire-resistance rating.

However, the performance of a fireproof coating depends not only on the product itself, but also on correct coating selection, surface preparation, application thickness, environmental conditions, and inspection procedures.

1. How to Select Fireproof Coatings for Steel Structures

Choose Between Indoor and Outdoor Fireproof Coatings

Indoor and outdoor fireproof coatings must not be used interchangeably.

Outdoor steel structures are exposed to rain, sunlight, wind, freezing temperatures, humidity, and repeated temperature changes. Therefore, exterior fireproof coatings should provide:

• Excellent water and moisture resistance

• Good freeze-thaw resistance

• Strong weathering and aging resistance

• Reliable adhesion and mechanical strength

• Compatibility with primers and protective topcoats

Using an indoor-only coating on an exterior steel structure may cause cracking, peeling, chalking, water absorption, and premature loss of fire protection.

Before selecting a product, confirm whether the steel members are located indoors, outdoors, in a humid area, or in a corrosive industrial environment.

Select Intumescent or Non-Intumescent Coatings

Steel structure fireproof coatings are generally divided into two categories.

Intumescent Fireproof Coatings

When exposed to fire, an intumescent coating expands and forms a thick carbonized insulating layer. It normally has a relatively thin dry-film thickness and provides a smooth, decorative appearance.

It is commonly used for:

• Exposed steel columns and beams

• Commercial buildings

• Airports and railway stations

• Exhibition centers

• Architectural steel structures

• Projects with appearance or space requirements

Non-Intumescent Fireproof Coatings

Non-intumescent coatings protect steel through a thicker, low-conductivity insulation layer. They are often selected for load-bearing members requiring a higher fire-resistance rating.

Typical applications include:

• Industrial plants

• Warehouses

• Power facilities

• Petrochemical buildings

• Concealed steel members

• Heavy load-bearing steel structures

For structural members requiring a fire-resistance rating above 1.5 hours, especially concealed load-bearing members, a tested non-intumescent system is generally preferred or may be required by the applicable project standard.

The required protection must be determined according to the product’s certified fire-test report. Simply increasing the coating thickness does not automatically guarantee a higher fire-resistance rating.

Do Not Use Decorative Fire-Retardant Coatings on Structural Steel

Decorative fire-retardant coatings and structural steel fireproof coatings serve different purposes.

Decorative fire-retardant coatings are mainly designed for wood, panels, and other combustible substrates. Their primary function is to reduce surface burning and slow flame spread. Their dry-film thickness is usually very low, and they generally cannot provide the thermal insulation needed to protect structural steel.

Only certified fireproof coating systems specifically designed and tested for structural steel should be used.

Check the Complete Tested Coating System

Fire resistance depends on the complete protective system, not only on the name of the coating. The following factors should be confirmed before application:

• Steel section size and section factor

• Required fire-resistance rating

• Specified dry-film thickness

• Indoor or outdoor service conditions

• Approved primer and topcoat

• Substrate preparation requirements

• Reinforcement mesh requirements

• Application method

• Environmental limitations

• Product certification and fire-test report

The selected primer, fireproof coating, and topcoat must be compatible. Using products from different systems without technical confirmation may result in poor adhesion, chemical reactions, cracking, or delamination.

2. Application Process for Steel Structure Fireproof Coatings

Step 1: Inspect and Prepare the Steel Surface

Before coating application, the steel surface should be clean, dry, and free from:

• Rust and mill scale

• Welding slag and spatter

• Oil, grease, and dust

• Loose paint

• Moisture and other contaminants

Surface preparation should be carried out according to the coating manufacturer’s instructions and project specifications. Damaged areas of the anticorrosive primer must be repaired before the fireproof coating is applied.

Step 2: Check Primer Compatibility

The existing primer must be compatible with the selected fireproof coating. Its adhesion, thickness, and surface condition should be inspected.

If the primer is damaged, contaminated, too smooth, or incompatible, corrective work may be required before the next coating layer is applied.

Step 3: Prepare the Fireproof Coating

The coating should be mixed strictly according to the manufacturer’s technical instructions.

For multi-component products:

• Use the specified mixing ratio

• Mix with suitable mechanical equipment

• Do not add excessive water or thinner

• Observe the recommended induction time

• Use the mixed material within its specified pot life

Improper mixing can reduce coating strength, adhesion, curing performance, and fire resistance.

Step 4: Apply the First Layer

The first coat should be applied evenly to establish reliable adhesion with the prepared steel surface.

Depending on the product, the coating may be applied by:

• Airless spray

• Specialized spraying equipment

• Brush

• Roller

• Trowel

The application method should match the coating type, steel member geometry, and manufacturer’s recommendations.

Step 5: Apply Multiple Coats to the Specified Thickness

Fireproof coatings should normally be applied in several layers. Excessively thick application in a single pass may cause sagging, cracking, incomplete curing, or delamination.

Each layer must be allowed to dry or cure sufficiently before the next coat is applied. Application should continue until the required dry-film thickness is achieved.

The final thickness should be based on:

• Required fire-resistance rating

• Steel section factor

• Product fire-test data

• Project design documents

• Manufacturer’s technical instructions

Step 6: Control the Application Environment

Temperature, humidity, ventilation, sunlight, wind, and condensation can all affect coating quality.

Application should be avoided under unsuitable conditions, including:

• Rain, snow, or fog

• Condensation on the steel surface

• Temperatures outside the product’s permitted range

• Excessively high humidity

• Strong wind

• Direct exposure to intense sunlight

During hot weather, rapid surface drying may cause cracking while the material underneath remains uncured. Shading, ventilation, and suitable working hours should therefore be considered.

Step 7: Inspect and Repair the Coating

After application, the coating should be inspected for:

• Dry-film thickness

• Surface uniformity

• Adhesion

• Cracks

• Sagging

• Hollow areas

• Delamination

• Missing or damaged sections

Areas that fail to meet the project requirements should be repaired according to the approved procedure.

Step 8: Apply the Protective Topcoat

For outdoor, humid, or corrosive environments, a compatible protective topcoat may be required to improve water resistance, weather resistance, chemical resistance, and long-term durability.

The topcoat should only be applied after the fireproof coating has fully dried or cured.

3. Causes and Repair of Cracks in Fireproof Coatings

Cracking is one of the most common quality problems in fireproof coating construction. Typical causes include:

• Applying the next layer before the previous layer has dried

• Applying an excessively thick coat in one pass

• Working under excessively high temperatures

• Rapid surface drying caused by direct sunlight

• Incorrect mixing ratio

• Excessive addition of water or thinner

• Poor substrate preparation

• Incompatible primer or topcoat

• Excessive vibration or movement of the steel structure

Minor surface defects should be assessed against the acceptance criteria of the project. Cracks exceeding the permitted width must be repaired.

A typical repair procedure includes:

1. Remove the cracked, loose, or poorly bonded coating.

2. Extend the removal area to sound and firmly bonded material.

3. Clean the exposed surface and remove all dust.

4. Repair damaged anticorrosive primer where necessary.

5. Reapply the fireproof coating in several controlled layers.

6. Allow each repair layer to dry before applying the next one.

7. Check the repaired area for thickness, adhesion, and appearance.

Simply covering a crack with another coat without removing defective material may conceal the problem rather than solve it.

4. Quality-Control Recommendations

To ensure reliable fire protection, contractors and inspectors should verify:

• Product certification and fire-test reports

• Correct coating type for the service environment

• Compatibility of the complete coating system

• Steel surface preparation

• Mixing ratio and pot life

• Application temperature and humidity

• Wet- and dry-film thickness

• Curing intervals between coats

• Final adhesion and surface condition

• Repair and inspection records

All work should follow the approved drawings, product technical data, fire-protection design, and applicable local building and fire-safety regulations.

Conclusion

The fire performance of a steel structure depends on selecting a properly tested coating system and applying it under controlled conditions. Indoor and outdoor products must be clearly distinguished, the required fire-resistance rating must match the certified coating system, and decorative fire-retardant coatings must never be substituted for structural steel fireproof coatings.

Correct surface preparation, multi-layer application, thickness control, curing, inspection, and defect repair are essential for providing durable and reliable fire protection throughout the service life of the steel structure.

At ZSJH Steel, we provide customized steel structure design, fabrication, surface treatment, export packaging, and technical support for industrial buildings, warehouses, workshops, and other steel construction projects. Send us your drawings and project requirements to receive a customized steel structure solution.