Now before you say ” I’m a DIYer this doesn’t apply ” hear me out, these points will help. A number of customers phone or email in with regards to preparation and talk about the amount of rust, failing coatings they’ve removed, but the important bit is this, how much have you left on the surface?
As a qualified Paint Inspector I’m looking at a surface and assessing whether the amount of contamination left on the surface is acceptable within the specification guidelines. For Instance if you are applying a Zinc Rich Epoxy Anode a quick wire brush won’t be good enough as this type of coatings requires a bright blast cleaned surface.
The standards for surface preparation, particularly for steel substrates, are critical in ensuring the longevity and performance of coatings applied to the metal. The standards Sa, St 2, and St 3 are part of the ISO 8501-1 standard, which is widely recognized in the industry. Each of these standards specifies a different level of surface cleanliness, achieved through various methods.
Sa Blasting Standards
The “Sa” designation refers to surface preparation achieved through abrasive blasting. These standards are part of the ISO 8501-1, which classifies the cleanliness of steel surfaces after abrasive blasting. The levels of Sa blasting are as follows:
- Sa 1 (Light Blast Cleaning):
- This is the lowest level of abrasive blasting. The surface is cleaned of loose mill scale, rust, and foreign matter. However, slight residues of contaminants may still be present, and the surface may still have stains from rust, mill scale, and old paint coatings.
- Sa 2 (Thorough Blast Cleaning):
- This standard requires more thorough cleaning. Most of the mill scale, rust, and foreign matter are removed. The surface may still have slight discoloration in the form of stains or shadows, but no significant amount of scale or rust should remain.
- Sa 2½ (Very Thorough Blast Cleaning):
- This is the most commonly specified standard for blast cleaning. Almost all mill scale, rust, and foreign matter are removed, leaving the surface uniformly greyish in color. This level of cleanliness is usually required for surfaces that will be coated with high-performance coatings.
- Sa 3 (Blast Cleaning to Visually Clean Steel):
- This is the highest level of cleanliness achievable by abrasive blasting. The steel surface appears visually clean, free of any stains, rust, scale, or other contaminants. The surface should be a uniform metallic color.
St 2 and St 3 Mechanical Cleaning Standards
The “St” standards refer to surface preparation achieved through hand and power tool cleaning. These standards are also part of ISO 8501-1, which outlines the quality of surface preparation using these methods.
- St 2 (Thorough Hand and Power Tool Cleaning):
- St 2 requires the removal of loose mill scale, rust, and poorly adhering paint by hand or power tools like wire brushes, scrapers, and grinders. After cleaning, the surface should have a uniform appearance, although some staining, slight rust, or tightly adherent mill scale may remain. St 2 is generally used for situations where less rigorous surface preparation is acceptable, such as in environments where the steel will not be exposed to highly corrosive conditions.
- St 3 (Very Thorough Hand and Power Tool Cleaning):
- St 3 is a step up from St 2 in terms of thoroughness. It requires the same types of tools but applied more rigorously to ensure that more of the surface contaminants are removed. The surface should be free from any loosely adhering mill scale, rust, or paint, and have a more pronounced metallic sheen compared to St 2. This level of cleaning is typically required in more demanding environments where better coating adhesion and durability are necessary.
Key Differences:
- Sa Standards involve abrasive blasting and are generally more effective and thorough than the St standards, which rely on mechanical tools. The Sa standards produce a much cleaner surface, which is often necessary for high-performance coatings.
- St 2 and St 3 standards involve hand and power tool cleaning, which are less aggressive methods than blasting. St 3 is more thorough than St 2, with a cleaner surface finish.
In summary:
- Sa 1 to Sa 3: Increasing levels of cleanliness achieved through abrasive blasting, with Sa 3 being the highest.
- St 2 and St 3: Standards for surface preparation using hand and power tools, with St 3 being more thorough than St 2.
These standards are crucial for ensuring that steel surfaces are properly prepared for painting or coating, which in turn, enhances the durability and performance of the coating.
