ISO 8501-1 Visual Cleanliness Grades

ISO 8501-1 is the international standard used to assess the visual cleanliness of steel surfaces before coating. Its full title is “Preparation of steel substrates before application of paints and related products: Visual assessment of surface cleanliness: Part 1: Rust grades and preparation grades of uncoated steel substrates and of steel substrates after overall removal of previous coatings.” It defines four rust grades for the starting condition of steel and several preparation grades for the result of blast cleaning, hand and power tool cleaning, and flame cleaning. Each grade is illustrated by representative photographs.

Why visual standards exist

Specifiers need a common language for how clean a surface must be. Written definitions alone leave room for interpretation, so ISO 8501-1 pairs written descriptions with photographs. The inspector compares the prepared surface with the photograph matching the specified grade and the original rust grade. Similar visual guides exist in North America (see SSPC/NACE surface prep standards).

Rust grades (initial condition)

Before preparation, steel is classified into one of four rust grades:

• A. Steel surface largely covered with adhering mill scale but with little, if any, rust.
• B. Steel surface that has begun to rust and from which mill scale has begun to flake.
• C. Steel surface on which the mill scale has rusted away or can be scraped off, but with slight pitting visible under normal vision.
• D. Steel surface on which the mill scale has rusted away and general pitting is visible under normal vision.

The starting grade matters because a surface blasted from grade D will look different from one blasted from grade A, even at the same preparation grade. Pitted steel may retain dark staining in pits.

Preparation grades

Blast cleaning (Sa)

• Sa 1, light blast cleaning. Removes loose mill scale, rust, paint and foreign matter.
• Sa 2, thorough blast cleaning. Most mill scale, rust and paint are removed; remaining contamination is firmly adhering.
• Sa 2½, very thorough blast cleaning. Any remaining traces of contamination show only as slight stains in the form of spots or stripes.
• Sa 3, blast cleaning to visually clean steel. Free from visible oil, grease, dirt, mill scale, rust, paint and foreign matter, with a uniform metallic color.

Sa 2½ is the most commonly specified grade for protective coatings on steel. Sa 3 is often required for thermal spray, some linings and immersion service.

Hand and power tool cleaning (St)

• St 2, thorough hand and power tool cleaning. Removes loose mill scale, rust, paint and foreign matter.
• St 3, very thorough hand and power tool cleaning. As St 2 but more thorough, giving a metallic sheen.

See hand tool cleaning and power tool cleaning.

Flame cleaning (Fl)

• Fl, flame cleaning. Removes mill scale, rust and paint by flame, followed by wire brushing. Rarely used today.

Approximate correspondence with SSPC and NACE

Specifiers often cross-reference grades. The commonly cited approximate equivalents are:

The definitions are not identical. For example, SSPC standards specify the maximum percentage of staining allowed per unit area, while ISO relies on the written description and photographs. Projects should use one system consistently. See AMPP, SSPC and NACE.

Related parts of ISO 8501

• ISO 8501-2 covers previously coated steel after localized removal of coatings.
• ISO 8501-3 covers preparation grades of welds, edges and other areas with imperfections (P1, P2, P3). See edge and weld preparation.
• ISO 8501-4 covers initial surface conditions, preparation grades and flash rust grades in connection with high-pressure water jetting. See water jetting.

What ISO 8501-1 does not cover

Visual grades do not measure everything that affects coating performance:

• Surface profile is measured separately under ISO 8503 (see surface profile measurement).
• Soluble salts are invisible and must be tested (see soluble salt testing methods).
• Dust is assessed by ISO 8502-3 (see dust contamination testing).
• Oil and grease may require additional checks.

A complete surface preparation specification covers all of these.

Using the photographs on site

• Use the current, undamaged edition of the standard. Photographs fade and cannot be photocopied accurately.
• View the surface and the photograph under good, even lighting.
• Compare with the photograph for the correct starting rust grade.
• Remember that abrasive type affects appearance. Steel grit and mineral abrasives can leave different colors at the same cleanliness.
• Record the result, location and lighting in daily inspection reports.

Key terms

• Rust grades A to D. The four initial conditions of steel in ISO 8501-1, from intact mill scale (A) to heavily pitted rust (D).
• Sa grades. Blast cleaning grades Sa 1, Sa 2, Sa 2½ and Sa 3.
• St grades. Hand and power tool grades St 2 and St 3.
• Fl grade. Flame cleaning, an older method rarely used today.
• Pictorial standards. Photographs that show each grade for each initial condition; the inspector compares the surface with the photograph.

Frequently asked questions

Is Sa 2½ the same as near-white?

They are commonly treated as equivalent, but definitions differ in detail. Follow the standard named in the contract.

Can flash rust be present at Sa 2½?

The grade describes the surface immediately after blasting. Flash rust that forms afterward is assessed separately, and many specifications require reblasting or limit its degree (see flash rust).

Is ISO 8501-1 enough for a specification?

No. It should be combined with profile, salt, dust and other requirements.

Related entries

• SSPC/NACE surface prep standards
• Abrasive blasting
• Mill scale
• Surface profile measurement
• Vacuum blasting (new)
• Rust and blister rating (ASTM D610 and D714) (new)