Method and Quality Grade of Surface Preparation

ISO 8504 parts 1 – 3 and ISO 12944 part 4 describe the methods used for metal surface preparation. Choosing the right/best method is generally decided on according to; working conditions; state of the metal surface; any known quality requirements of the surface preparation; full or partial treatments; financial factors; and any special requirements or process constraints. Underlying however, the surface preparation requirements are determined based on the paint or coating system to be applied.

Grease and Surface Soils

Surface contamination of grease and surface soils will make the removal of rust a more difficult process to successfully complete because of the masking effects they have. ISO 12944-4 provides methods for removal of such contaminants. Solids such as paint residues, plaster and such like are generally removed by scraping or scrubbing the surface. Surface salts and other water-soluble contaminants are removed by washing and scrubbing, or in some cases high-pressure jet-washing or even steam treatment and in some cases washing with an alkali solution then rinsing.

Rust (Oxidation)

Once the grease, oils and other soils have been cleaned away it’s now possible to remove any surface oxidation and mill scale. This may be a mechanical process, thermal or chemical process method.

Wire brushing (St) is performed using hand tools such as wire brush or grinder hand tools – ISO 8504-3

Abrasive blast cleaning (Sa) uses a granular media ‘blasted’ at the metal surface using pressurised air or water or centrifugal systems – ISO 8504-2

Hydro-blasting (Wa) uses ultra-high pressure water at more than 70 MPa, the energy of the water being enough to prepare the metal surface – ISO 8501-4 and SSPC VIC-4

Thermal removal of mill scale is by flame cleaning where acetylene-oxygen flame is applied to remove old pant layers, mill scale and rust, followed by cleaning with a wire brush – ISO 9501-1

Chemical rust removal typically involves acid pickling where the mill scale and rust is dissolved by the acid mixture


The first thing to do is assess the state of the metal surface before any preparation work is carried out and record the information as a ‘rust grade’. The state of the metal surface immediately after preparation is then recorded as the preparation quality grade.

Rust Grades

Hot milled steel will present with a layer of mill scale and any evidence of rusting will be influenced by time and exposure to environmental factors. ISO 8501-1 specifies four rust grades designated as A, B, C, or D and are defined along with illustrative example photographs.

Grade A for example will be largely mill scale still in situ with a minimal level of rust

Grade B is where the mill scale has begun to flake off and the initial signs of rust showing

Grade C is where the mill scale has largely rusted away

Grade D means that the metal is rusted and is pitted also

Preparation Grades

ISO 8501-1 provides surface preparation grades, again with pictorial examples of the texture of the surface after preparation activities.

Surface preparation by hand tools is indicated by ‘St’ followed by a number indicating the degree of cleanliness. So for example St 2 would mean careful cleaning by hand held tools and the result being the metal surface shows no signs of dust, grease, oil, weakly adherent mill scale, rust paint or other substances.

Surface preparation by blast cleaning is indicated by ‘Sa’ with a number, i.e. Sa 1, Sa 2, S2 ½ and Sa 3. Sa 2 ½ means very careful blast cleaning of the metal substrate, the surface must not contain any dust, grease, oil, mill scale, paint or other substances.

Hydro-blast cleaning is a method using only water under high pressure. The aim being to remove soluble salts, oil, grease, dust. But this method does not remove mill scale and does not leave a surface profile to increase the surface area profile.

Surface Profile Grades

Surface profile refers to the micro-roughness of the metal surface after preparation treatments and is the ratio of the highest profile peak to the lowest profile valley as determined by ISO 8503-1. The nature of the blast cleaning process means that random irregularities are inevitable, so it’s noted and generally accepted that no method of measurement will provide a precise measurement value. Instead profiles are described as ‘dimpled’ where shot abrasives have been employed or ‘angular’ where grit abrasives have been employed.

Grading according to ISO 8503-1

Fine Grade – profiles in line with segment 1 and up to 2

Medium Grade – profiles in line with segment 2 up to 3

Course Grade – profiles in line with segment 3 up to 4

Grit Blast Cleaning ‘G’ Shot Blast Cleaning ‘S’
Segment Nominal Reading Tolerance Segment Nominal Reading Tolerance
1 25 µm 3 µm 1 25 µm 3 µm
2 60 µm 10 µm 2 40 µm 5 µm
3 100 µm 15 µm 3 70 µm 10 µm
4 150 µm 20 µm 4 100 µm 15 µm


About Northpoint

Over the years Northpoint has developed organisational skills and expertise that go beyond their powder coating roots (although powder coating remains just one of the many services Northpoint provide to industry) which include being competent in meeting the requirements for DWI (Drinking Water Inspectorate) coating requirements, NORSOK M-501 coating requirement for use in both on and off-shore applications, SaudiAramco, KOC, Petrofac and more paint specifications typically specified for petro-chemical processing applications.

Northpoint are well versed in FBE (Fusion Bonded Epoxy) coatings and systems that use FBE as a part of the corrosion protection coating system employed.  For example, FBE coatings that are used as an outer layer with solar protective coatings applied over the top with a thixotropic system applied to an internal surface. The coating systems allow for pipework to be welded on site without the need for people to enter into the pipe, but a continuous protective coating layer provided.  The net result being expensive flanged connections are avoided, and site safety is enhanced. To get to this point took several meetings both virtually and face-to-face, a PQP/ITP (Project Quality Plan and Inspection Test Plan) and a Third Party Audit of the processes.

To provide customers and clients with the very best service ensuring that specifications are understood, interpreted correctly, and delivered on Northpoint has taken the decision to invest heavily in its people. On staff, there is a NACE III Paint and Coatings Inspector who supervises the activities of two further NACE II qualified Inspectors. Add to this that Northpoint also has as two team members ISO 9001 QMS, 14001 EMS, ISO 45001 OHSMS & 27001 ISMS Qualified Lead Auditors and two ISO 9001 QMS Internal Auditors as well as qualified paint applications operative team members and Northpoint has the knowledge, experience and understanding to deliver coating and painting services that will meet customers’ demands and needs.

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Tel:         0161 330 4551   (+44 161 330 4551)


Blog Author – Philip Dawson, Technical Advisor, for technical and specification enquiries