ISO 8501-1
Preparation of Steel Substrates Before Application of Paints and Related Products — Visual Assessment of Surface Cleanliness — Part 1
Defines rust grades and visual preparation grades for uncoated steel and steel after complete removal of previous coatings.
What ISO 8501-1 specifies
ISO 8501-1 is the visual cleanliness reference used before painting structural steel. It establishes initial rust grades A through D and preparation grades for blast cleaning Sa 1, Sa 2, Sa 2½, and Sa 3, plus hand and power-tool cleaning grades St 2 and St 3. Sa 2½ is the common very-thorough blast-cleaning requirement; Sa 3 is visually clean steel with a uniform metallic appearance. The grades describe visible contamination removal, not anchor-profile depth, soluble salts, dust, or hidden contamination. A coating specification should pair the cleanliness grade with a separate surface-profile and contamination requirement.
ISO 8501-1 types and classes
Every designation the document defines, with the thickness it requires and the tests it imposes.
| Designation | Meaning | Requirement / test |
|---|---|---|
Sa 1 | Light blast cleaning; loose mill scale, rust, paint, and foreign matter removed | Visual comparison with ISO photographic reference |
Sa 2 | Thorough blast cleaning with most contamination removed | Visual comparison with ISO photographic reference |
Sa 2½ | Very thorough blast cleaning; remaining traces appear only as slight stains | Visual comparison; broadly associated with SSPC-SP 10/NACE No. 2 |
Sa 3 | Blast cleaning to visually clean steel with uniform metallic appearance | Visual comparison; broadly associated with SSPC-SP 5/NACE No. 1 |
St 2 | Thorough hand and power-tool cleaning | Visual comparison with ISO photographic reference |
St 3 | Very thorough hand and power-tool cleaning producing a more pronounced metallic sheen | Visual comparison with ISO photographic reference |
Tests and the drawing call-out
Tests the document imposes
- Visual rust-grade assessment A–D
- Visual blast-cleaning grade Sa 1–Sa 3
- Visual hand and power-tool grade St 2–St 3
How to cite it
Drawing call-out
BLAST CLEAN STEEL TO ISO 8501-1 Sa <1|2|2½|3>; SURFACE PROFILE <RANGE>Angle brackets mark the choices the drawing must make; every finish page shows a completed example.
ISO 8501-1 — frequently asked
What is ISO 8501-1 Sa 2.5?
Sa 2½ is very thorough blast cleaning; visible contamination is removed and remaining traces appear only as slight stains.
Is Sa 3 the same as SSPC-SP 5?
They are commonly treated as broadly comparable visually clean or white-metal blast-cleaning requirements, but contracts should cite the governing standard explicitly.
Does ISO 8501-1 specify surface profile?
No. It grades visible cleanliness. Profile depth and soluble contamination need separate requirements.
ISO 8501-1 is copyrighted by International Organization for Standardization. This page explains the document so a drawing can be read; it does not reproduce the standard. Purchase the current edition from the publisher. Official source ↗