Blast cleaning does more than clean steel. It roughens the surface, creating the profile (sometimes called the anchor pattern) that a coating grips. ISO 8503 describes how that profile is graded and measured.
Why profile matters
- Too smooth, and the coating has little to key into, so adhesion suffers.
- Too rough, and the peaks can poke through a thin first coat, leaving rust spots and pinholes. A rough surface also needs more paint to cover it.
Most coating data sheets state a required profile range, and the specification should repeat it.
Grading with a comparator: ISO 8503-1 and 8503-2
A comparator is a reference plate with segments of known roughness. The inspector compares the blasted surface with it by sight and touch. There are two types: G for grit-blasted surfaces (angular profile) and S for shot-blasted surfaces (rounded profile).
For the grit (G) comparator the grades are commonly described as:
- Fine: roughly 25 to 60 µm
- Medium: roughly 60 to 100 µm
- Coarse: roughly 100 to 150 µm
Measuring profile
Where a number is needed, profile can be measured with:
- Replica tape (ISO 8503-5): a compressible film pressed onto the surface and measured with a micrometer.
- Stylus instruments (ISO 8503-4): a fine stylus traces the surface to give a roughness reading.
Profile and film thickness
Profile also affects dry film thickness readings. On a rough surface, a gauge calibrated on smooth steel reads high, which is why ISO 19840 applies correction values based on the profile grade.
How we help
We check profile as part of surface preparation inspection, and help specify the right abrasive and profile for the coating system. See our inspection service.
This guide summarises the standard for general understanding. Always work to the current edition and the project specification.