# ISO 8501

Source: https://coatmentor.com/standards/iso-8501/ · Updated 2026-10-04

Visual assessment of steel surface cleanliness: rust grades and preparation grades.

**Publisher**: ISO

**Area**: Surface preparation and conditions

ISO 8501 is the visual reference for how clean prepared steel must be. The inspector compares the surface with the written definitions and the photographs, in good light, without magnification.

## The parts an inspector meets

- **Part 1:** rust grades A to D of uncoated steel, and the preparation grades Sa 1, Sa 2, Sa 2½ and Sa 3 (blast cleaning), St 2 and St 3 (hand and power tool cleaning) and Fl (flame cleaning).
- **Part 2:** grades for previously coated steel after the coating is removed in places (P Sa, P St, P Ma).
- **Part 3:** preparation grades P1, P2 and P3 for welds, edges and other imperfections.
- **Part 4:** the initial surface condition, preparation grades and flash rust grades for high-pressure waterjetting.

## In practice

A specification usually names a grade such as Sa 2½. The photographs show the grade for each starting rust grade. The inspector first records the rust grade, then compares like with like. The SSPC/NACE grades are close counterparts, not identical definitions.

**47** practice items in the members area mention ISO 8501 (CIP 1: 20 · CIP 2: 13 · FROSIO written: 14).

## Related

- **Concepts:** [Surface cleanliness grades](https://coatmentor.com/learn/surface-cleanliness/) · [Waterjetting and flash rust](https://coatmentor.com/learn/waterjetting/)
- **Standards:** [SSPC-SP, NACE No. and WJ](https://coatmentor.com/standards/sspc-surface-preparation/) · [ISO 8502](https://coatmentor.com/standards/iso-8502/) · [ISO 8503](https://coatmentor.com/standards/iso-8503/)
- **Defects:** [Flash rusting](https://coatmentor.com/defects/flash-rusting/) · [Edge corrosion (thin film at edges)](https://coatmentor.com/defects/edge-corrosion/) · [Steel surface imperfections](https://coatmentor.com/defects/steel-surface-imperfections/)

## About this page

This page describes the subject of ISO 8501 in our own words; it does not reproduce the standard. For the full text, buy the current edition from ISO. Checked on 2026-10-04; errors go in the [corrections log](https://coatmentor.com/method/). CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.