Surface Preparation of Blasted Steel: Why Surface Profile and Cleanliness Matter

Achieving long-term coating performance begins with proper surface preparation. No coating system regardless of its chemistry or cost, can compensate for an inadequately prepared surface. Within this process two parameters are often considered; Surface Profile and Surface Cleanliness. Throughout this blog we’ll further define and explore the steps required to successfully achieve surface profile and surface cleanliness. 

 

Understanding Surface Profile

A surface profile is the texture created on the steel after abrasive blasting. This microscopic pattern of peaks and valleys (often called an anchor pattern) allows the coating to mechanically bond with the substrate.

The correct surface profile depends on the coating specification and is typically expressed as an average peak-to-valley height in microns (µm).

 

How to Measure Surface Profile

Two common tools for checking surface roughness are:

• Elcometer 122 Testex Replica Tape – a simple, reliable way to capture surface impressions.
• Elcometer 224 Digital Surface Profile Gauge – a fast, digital alternative ideal for field use.

 

How to measure Surface Profile using an Elcometer 224 Surface Profile Gauge:

1. Calibrate the gauge on a glass zero tile and hold the probe at 90° to the surface.
2. Take 5–10 readings within a 100 mm (4”) area.
3. Depending on the mode, record either the average or maximum peak-to-valley height.
4. For larger surfaces, take multiple spot measurements (e.g., 5 per 1 m² area).

Watch the process: Understanding Surface Profile – Elcometer

 

Factors Influcing Surface Profile

• Abrasive type (e.g. Garnet, Glass, Steel Grit)
• Abrasive size and hardness
• Nozzle pressure and standoff distance (when blasting)

 

A consistent and correct surface profile ensures optimal mechanical adhesion between coating and substrate.   But getting the balance right is key; excessively low profile can lead to poor adhesion, meanwhile excessively high profile may result in premature rusting due to the surface not being coated correctly.

 

Surface Cleanliness

Surface cleanliness defines the degree of contaminant removal achieved during blasting; including rust, mill scale, previous coatings, oil, grease, salts, and other foreign matter

Cleanliness is assessed visually and chemically, using standards such as:

• ISO 8501-1 – Visual assessment
• AS/NZS 1627.4 – Preparation of steel surfaces prior to painting

 

Typical Cleanliness Grades (AS 1627.4)

Description	Australian Standard
Brush Blast	AS1627.4 Class SA 1
Commercial Blast	AS1627.4 Class SA 2
Near White Blast	AS1627.4 Class SA 2.5
White Blast	AS1627.4 Class SA 3

 Reference: Elcometer SSPC-VIS 1 Guide and Photographs

 

Verification tools and checks may include:

• Chloride contamination testing (Bresle method – ISO 8502-9)  -  Elcometer 130 Salt Contamination Meter 

• Dust contamination levels (ISO 8502-3)  -  Elcometer 142 Dust Tape Test Kit 

• Surface comparators and visual reference photographs  -  Elcometer 125 Surface Comparators 

 

The Relationship Between Profile and Cleanliness

The two parameters of surface profile and cleanliness are distinctly interdependent, yet they are at the same time closely related. It is possible to achieve the correct surface profile yet still fail the cleanliness requirements; for example, when embedded abrasive dust or salt contamination remains after blasting.

To ensure compliance:

• Verify cleanliness first, then confirm profile height
• Use clean, dry abrasive (check for low salt content with the Elcometer 134A Chloride Ion Test Kit).
• Maintain controlled environmental conditions; with substrate temperature ≥ 3 °C above dew point, and relative humidity below 85%.

 

In summary, surface preparation is the foundation of any successful coating system. Both surface profile and cleanliness must meet specified standards to guarantee coating system performance and service life.  Neglecting either can lead to; early coating failure, accelerated corrosion and reduce coating longevity. Routine substrate inspections, environmental monitoring, and documentation during preparation are essential for quality assurance and lasting coating performance.

 

At Spray Blast Inspect, we supply a comprehensive range of Elcometer instruments and inspection tools designed to help applicators achieve specification-compliant results every time.  Alternatively, our team of accredited NACE Coating Inspectors can support you with all aspects of substrate and coating inspections, ensuring surface preparation and coating performance meet industry standards.

 

Contact our SBI Coatings experts today.

 

written by Darryl Burge