Surface preparation can account for up to 40 percent of structural steel painting and repainting jobs and the life of anti‐corrosion coatings on a steel surface largely depend on how thoroughly the surface was prepared before painting.
At Rosler, we have extensive experience evaluating structural steel surfaces for coating before and after shot blasting. This knowledge of surface preparation standards and the widely used ISO and SSPC standards guide us in developing systems to expertly prepare and repair structural steel throughout its lifespan.
Evaluating rust and mill scale pre- and post-shot blasting is a must. It is important to clearly specify the quality of the surface prior to preparation as well as the surface conditions after preparation. As a result, standards were developed to visually assess the initial surface conditions and the quality of the required surface preparation relative to the initial steel surface conditions.
The dominant and widely used standards for evaluating rust and mill scale are ISO 8501‐1:2007 (based on the Swedish Standard SIS 05 59 00) and the SSPC (Steel Structures Painting Council). While different in some minor details, these standards are practically identical.
In specifications relating to the preparation of surfaces prior to painting, the SSPC and SIS designations correspond as follows.
Corresponding SSPC and SIS designations
The standards identify four initial rust grades based on visual evaluation before surface treatment. The following examples illustrate and describe the four ratings.
Visual comparison of rust grades
- A – The steel surface is covered completely with adherent mill scale and little, if any, rust.
- B – The steel surface has begun to rust and mill scale has begun to flake from the surface.
- C – The steel surface on which the mill scale has rusted away or from which it can be scraped, shows little pitting visible to the naked eye.
- D – The steel surface on which mill scale has rusted away shows considerable pitting visible to the naked eye.
After blast cleaning, the standards define four preparation grades. The appearance of the shot blasted surface must correspond to one of the following descriptions and standards:
The cost to achieve longevity of each grade varies and should be considered based on initial cost as well as lifetime costs based on your structural steel’s uses and expected lifespan.
- Brush‐off – After light blast cleaning loose mill scale, rust, and foreign matter are removed. Despite being the least expensive grade of the four, brush-off preparation may not hold paint and coatings as well as the other grades and often requires more frequent resurfacing.
- Commercial – Thorough blast cleaning removes almost all mill scale, rust, and foreign matter. While normally adequate in a non-corrosive atmosphere, this grade is a more labor- and cost-intensive option compared to brush-off preparation.
- Near White – Very thorough blast cleaning removes mill scale, rust, and foreign matter to the extent that the only remaining traces are light stains in the form of spots and stripes. This grade is fully acceptable for most somewhat-corrosive environments. It is the most common preparation grade for ship building and most industrial and commercial weldments and fabrications.
- White Metal – Blast cleaning reveals pure metal. This grade is very expensive and typically reserved for applications in which the costs of coating failure are catastrophic, for example, for tanks containing highly corrosive, hazardous chemicals.
Preparation Grade Examples Based on ISO 850-1
The different preparation grades depend on the initial rust grade. ISO 850-1 standard grades are presented below to provide visual examples. SSPC Vis 1/89 incorporates similar visual comparisons.
Visual comparison of preparation grades
The Rosler Way
Whatever your pre- and post-treatment goals, count on Rosler to help you find a better way.