ISO 8501-1 is the visual standard for how clean steel must be before it is painted. It uses reference photographs so an inspector can compare the prepared surface with an agreed grade. Almost every protective coating specification refers to it.
Rust grades: the starting condition
Before preparation, the steel’s condition is described by one of four rust grades:
- A: covered in adherent mill scale with little or no rust
- B: has begun to rust and the mill scale has begun to flake
- C: the mill scale has rusted away or can be scraped off, with slight pitting
- D: the mill scale has rusted away and pitting is widespread
The starting grade matters because the same preparation grade looks different on each. That is why grades are written as, for example, B Sa 2½.
Blast cleaning grades
- Sa 1, light blast cleaning: loose mill scale, rust and paint removed.
- Sa 2, thorough blast cleaning: most mill scale, rust and paint removed; what remains is firmly adhering.
- Sa 2½, very thorough blast cleaning: only slight traces remain, as spots or stripes. The most commonly specified grade for protective systems.
- Sa 3, blast cleaning to visually clean steel: a uniform metallic colour with no traces at all.
Hand and power tool grades
Where blasting is not possible, surfaces may be prepared with hand or power tools:
- St 2: thorough hand and power tool cleaning
- St 3: very thorough, with a metallic sheen from the substrate
Most high-performance coatings need a blast-cleaned surface. Using a system over St grades that its data sheet does not allow is a common cause of early failure.
What the grade does not tell you
ISO 8501-1 is about visible cleanliness only. A surface can look perfect at Sa 2½ and still carry invisible soluble salts, dust or the wrong profile. Those are covered by ISO 8502 and ISO 8503, and a good inspection checks all three.
How we help
We assess preparation grades on site against the reference photographs, record the result and flag problems before the primer goes on, when fixing them costs the least. Talk to us about inspection.
This guide summarises the standard for general understanding. Always work to the current edition and the project specification.