Surface preparation standards are stipulated by various bodies throughout the world to designate the cleanliness condition of blasted steel, prior to applying a protective coating.
The applicable cleanliness standard is usually called for by either the protective coating manufacturer or the owner of the structure to be painted.
The most commonly referred to standards are SSPC, NACE, and Swedish Standards.
Each standard is divided into four standards of cleanliness, broadly described as follows; brush off, commercial, near white metal, white metal. Whilst each standard may differ slightly in requirements and terminology the following cross reference chart indicates the close approximation of each level of cleanliness for each standard.
| BRUSH OFF | COMMERCIAL | NEAR WHITE METAL | WHITE METAL | |
|---|---|---|---|---|
| SSPC | SP 7 | SP 6 | SP 10 | SP 5 |
| NACE | No.4 | No.3 | No.2 | No.1 |
| SWEDISH | Sa.1 | Sa.2 | Sa.2.5 | Sa.3 |
A brief description of each of the four levels of cleanliness is as follows:
Brush Off - Loose mill scale, loose rust and foreign particles are removed.
Commercial - Mill scale, rust and foreign particles are substantially removed and grey metal is visible.
Near White Metal - Mill scale, rust and foreign particles are removed to the extent that only traces remain in the form of spots or stripes. The cleaned surface will show varying shades of grey.
White Metal - Visible mill scale, rust and foreign particles are entirely removed. The cleaned surface should have a uniform metallic colour but may show varying shades of grey when viewed at different angles.
The full standards are available from each organisation from the following links:
Steel Structures Painting Council
National Association of Corrosion Engineers
International Organization for Standarization
Swedish Institute for Standards
Standards Australia
The appearance of steel after blasting will vary greatly and is subject to, amongst other things, the condition and appearance of the steel prior to blasting. Other factors that influence the post blasting appearance include, type and size of abrasive used, angle of blast and profile height.
The intial condition of the steel, prior to blasting, can be divided into one of four categories: (As per AS1627.4-1989)
Condition A. Steel surface covered completely with adherent mill scale and with little if any, rust.
Condition B. Steel surface which has begun to rust and from which the mill scale has begun to flake.
Condition C. Steel surface on which the mill scale has rusted away, or from which the mill scale can be scraped, but having little pitting visible to the naked eye.
Condition D. Steel surface on which the mill scale has rusted away, and on which considerable pitting is visible to the naked eye.
The following pictorial table illustrates the variations in surface appearance after blasting, depending on the intial condition of the steel prior to blasting.
Condition A Unblasted |
Condition A Blasted Sa.1 |
Condition A Blasted Sa.2 |
Condition A Blasted Sa.2.5 |
Condition A Blasted Sa.3 |
Condition B Unblasted |
Condition B Blasted Sa.1 |
Condition B Blasted Sa.2 |
Condition B Blasted Sa.2.5 |
Condition B Blasted Sa.3 |
Condition C Unblasted |
Condition C Blasted Sa.1 |
Condition C Blasted Sa.2 |
Condition C Blasted Sa.2.5 |
Condition C Blasted Sa.3 |
Condition D Unblasted |
Condition D Blasted Sa.1 |
Condition D Blasted Sa.2 |
Condition D Blasted Sa.2.5 |
Condition D Blasted Sa.3 |
