Abrasive Blasting to SA 2.5 (Near-White Metal Finish)

Abrasive blasting to SA 2.5 removes rust, mill scale and failed coatings to expose clean steel substrate. This preparation creates the mechanical key required for intumescent coating adhesion and verified fire-rating performance. For many intumescent systems, SA 2.5 preparation is a mandatory requirement for manufacturer warranty and building control approval.

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Understanding the SA 2.5 Standard

What “Near-White Metal” Means

SA 2.5 is defined under ISO 8501-1 as a near-white metal finish where no more than 5% of the surface shows light staining, shadows or residual marks when viewed under standard lighting conditions. This benchmark ensures the steel is sufficiently clean to support long-term adhesion and reliable intumescent performance under fire exposure.

When SA 2.5?

When SA 2.5 Preparation Is Required

Abrasive blasting to SA 2.5 is specified where surface condition or risk level demands full removal of contaminants.

Heavy Corrosion

Extensive rust and corrosion products require complete abrasive removal to restore surface integrity.

Failed Coatings

Delaminated, peeling or incompatible coatings must be stripped back to bare steel to prevent adhesion failure.

Deep Corrosion Pitting

Significant surface loss requires uniform blast cleaning to create a consistent profile across all steel elements.

Mill Scale

Factory-formed oxide layers inhibit adhesion and must be fully removed for compliant coating application.

Adhesion & Mechanical Key

Why Surface Profile Matters

Abrasive blasting creates a controlled surface profile that allows intumescent coatings to bond mechanically to the steel. This mechanical key prevents delamination under thermal stress and supports long-term system durability. Without the correct profile, coatings can fail prematurely regardless of product quality or applied thickness.

Blasting Equipment & Media

Consistent Preparation Using Industrial Systems

SA 2.5 preparation is carried out using industrial-grade blasting equipment to maintain consistent pressure, coverage and surface profile.

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01

High-Capacity Air Supply

Compressors deliver sustained air volume and pressure throughout the blast cycle to ensure uniform cleaning.

02

Abrasive Media Selection

Garnet and steel grit are selected based on surface condition and project requirements. Garnet provides sharp cutting action, while steel grit offers durability and efficiency for large-scale works.

Verification of SA 2.5 Compliance; Inspection Before Coating Application

SA 2.5 compliance is verified through visual comparison against ISO 8501-1 standards and supported by dust and cleanliness checks where required. Verification confirms the substrate is suitable before any primer or intumescent coating is applied, preventing costly failures later in the process.

Impact on Fire-Protection Performance

Preparation Enables Accurate WFT Control

A clean SA 2.5 surface allows intumescent coatings to be applied evenly and measured accurately. This directly supports the achievement of specified Wet Film Thickness targets and ensures the installed system matches tested fire-performance data.

Eliminates voids and uneven coating build
Enables consistent WFT measurement across steel sections
Reduces risk of premature delamination and corrosion

Containment & Site Control

Managing Dust & Environmental Impact

Abrasive blasting generates dust and debris and must be carried out within controlled containment. Enclosures, shrouded equipment and vacuum systems are used to limit airborne particles, protect surrounding areas and maintain safe working conditions on live sites.

Benefits and Practical Considerations

Why SA 2.5 Is the Gold Standard

Advantages

SA 2.5 preparation guarantees manufacturer compliance, supports certification and delivers maximum system durability under thermal and environmental stress.

Limitations

Blasting requires specialist equipment, trained operators and controlled conditions. Noise, weather sensitivity, waste disposal and programme duration must be considered during planning.

When SA 2.5 Is Non-Negotiable

Intumescent coating manufacturers frequently specify SA 2.5 preparation as a mandatory requirement for warranty and fire-rating certification. Fire engineers and building control bodies require documented evidence of SA 2.5 preparation before approving fire-protection systems for higher-risk structural steel applications.