Surface preparation standards explained-SSPC/NACE & ISO 8501

The two dominant abrasive blast cleaning standards, ISO 8501 and the SSPC/NACE joint standards, are tough to compare. Although they recognize roughly the same levels of cleanliness, they classify them in opposite ways, muddying the water.
 

ISO 8501 was published by the International Standards Organisation in 1988, after combining the content from the 1967 Swedish Standard SIS 055900 with the German DIN 55928. ISO 8501 is a pictorial standard showing the appearance of different rust grades at various levels of cleanliness, although it also contains text descriptions of the cleanliness levels. ISO 8501 ranks cleanliness levels in order of increasing work required.

Sa 1 Light Blast Cleaning
Sa 2 Thorough Blast Cleaning
Sa 3 Blast Cleaning to Visually Clean Steel
 

In North America, the original surface prep standard was written by an architectural group in the 1960s for steel workers in Pittsburgh. The Society for Steel Painting Structures formed around the standards. The SSPC standards are text descriptions, not pictorial, although they are accompanied by visual guides (VIS) with photo references. SSPC/NACE numbers them in reverse order, by increasing surface cleanliness.

SP 5 White Metal
SP 6 Commercial
SP 7 Brush Off
 

This was straightforward enough until the industry demanded a new specification for a cleanliness grade that could cut costs by replacing White Metal in situations where near-white was good enough. ISO adapted it into their established order as Sa 2 1/2, Very Thorough Blast Cleaning, but SSPC went outside the order, adding it chronologically as SP 10 Near White. The two versions were not equal: Sa 2.5 permitted stains, streaks and shadows from rust, mill scale and coatings to remain on up to 15%* of the surface, whereas SP 10 allowed for only 5%.

In 2000, SSPC and NACE issued joint standards in anticipation of a merger between the organisations, whose memberships largely overlapped. The merger fell through, but NACE’s new order, which mapped NACE No. 1, 2, 3, 4 onto SSPC SP 5, 6, 10, 7, persisted.

This cleared things up until 2006 when SSPC/NACE introduced Industrial Blast Cleaning, a new specification between Brush Off and Commercial, and classified it chronologically as SSPC SP 14 / NACE No.8. ISO declined to shoe-horn the new specification into their system as Sa 1.5.

Despite the differences, the grades of cleanliness are generally thought to be compatible. They reflect similar permissible levels of stains and tightly-adhered rust, mill scale and coatings, and can be summed up with a chart:

*Estimated surface area. ISO 8501-1 is a visual reference and does not explicitly state percentages.

Surface preparations standards exist to maximize coating life and minimize costs.

With surface prep accounting for up to 40% of the cost of a repainting project, facility owners look to limit the material and hours spent blasting. White Metal is expensive to achieve, especially on maintenance jobs, and typically reserved for critical applications where the cost of failure is catastrophic. Near white is good enough for service in most severe environments. Commercial is less expensive and suitable for non-corrosive atmospheres and service environments. Brush Off will save the owner the most money in the short run, if he can get away with it.

When choosing a coating, the owner weighs the costs of blasting and painting against the risk of a premature coating failure. If the worst-case scenario is that he has to repaint in 5 years instead of 7, he might save money by cutting back from Commercial to Brush-Off. When premature coating failure could result in the spilling of five million gallons of corrosive, hazardous and expensive chemical, he’ll lean towards White Metal and a high performance coating.