One of the most frequently encountered expressions in paint specifications is the request for "Sa 2½ blasting." This expression does not define a method but rather the visual cleanliness level the surface must achieve after blasting. The reference document is ISO 8501-1.
Initial rust grades
ISO 8501-1 first defines the initial condition of steel. This classification indicates how deteriorated the surface is before blasting and determines the workload required to achieve the target cleanliness degree.
- A: Mill scale is largely adherent to the surface, with negligible rust.
- B: Mill scale has begun to lift, and rust is visible on the surface.
- C: Mill scale has largely lifted, with widespread rust; pitting is limited.
- D: Mill scale has lifted, with widespread rust and visible pitting.
Abrasive blasting grades
The cleanliness levels achieved by blasting are indicated by the letter Sa. As the degree increases, the amount of residue allowed to remain on the surface decreases, consequently increasing processing time and abrasive consumption.
- Sa 1: Light sweep blasting. Loose mill scale, rust, and coatings are removed.
- Sa 2: A large portion of the surface is cleaned, with limited residue allowed.
- Sa 2½: Very thorough blasting. Mill scale, rust, old coatings, and foreign matter are almost entirely removed; only slight traces may remain.
- Sa 3: Visually clean steel. The surface achieves a uniform metallic appearance.
Cleaning with hand and power tools
St grades are used when blasting cannot be applied. St 2 defines thorough cleaning with hand or power tools, while St 3 defines more meticulous cleaning. These methods do not create the same surface profile as blasting; therefore, the adhesion expectations of the paint system should be evaluated accordingly.
SSPC and NACE equivalents
If the specification comes from an international client, the cleanliness level may be specified with SSPC or NACE/AMPP descriptions. Although these descriptions are not exactly the same as ISO degrees, they are commonly matched. The exact correspondence is determined by which standard is taken as the basis in the contract.
- SP 5 (White Metal) approximately corresponds to Sa 3 level.
- SP 10 (Near-White) approximately corresponds to Sa 2½ level.
- SP 6 (Commercial) approximately corresponds to Sa 2 level.
- SP 7 (Brush-Off) approximately corresponds to Sa 1 level.
Cleaning alone is not sufficient
The surface is not expected to be merely clean after blasting. A controlled surface profile is also required for the paint system to adhere; this profile is evaluated with reference to ISO 8503. Furthermore, the levels of dust, salt, and chlorides remaining on the surface are checked under ISO 8502.
Blasted surfaces re-oxidize rapidly in open air. Therefore, the surface must be taken into the painting process as soon as possible, protected from dust, moisture, and the risk of flash rust. Conducting blasting and painting in the same facility shortens this waiting period.
Bu yazı genel bilgilendirme amaçlıdır. Her projede sözleşme, müşteri şartnamesi, boya üreticisi dokümanları ve güncel standart baskıları esas alınır.