Properly preparing a steel surface for coatings requires far more than a quick clean. In fact, there are international standards that should be met to ensure the best chance of adhesion for primers and coatings applied post-clean. These preparation methods must be suitable enough to remove dust, old paint, and other foreign matter to leave behind a smooth surface for coatings to grip.
From manually removing rust with mechanical grinders to utilising abrasive blast cleaning, a professional team must use the right tools for the job while continuously assessing their work to ensure the correct preparation grades are met.
In this article, we’ll explore these preparation grades and methods, and the international standards they’re aligned with.
Understanding the BS EN ISO 8501 standard series
BS EN ISO 8501 and its four parts are the ISO standards for steel surface preparation before the application of coatings, paints, and similar products.
This international standard series is a visual assessment of how clean the steel surface is, covering rust grades and preparation grades. Teams like ours need to use this standard to ensure steel surfaces are in the right condition for proper coating adhesion.
ISO 8501 is split into four parts (the following descriptions can be found on the BSI website):
- ISO 8501-1 – Rust grades and preparation grades of uncoated steel substrates and of steel substrates after overall removal of previous coatings
- ISO 8501-2– Preparation grades of previously coated steel substrates after localised removal of previous coatings
- ISO 8501-3 – Preparation grades of welds, edges and other areas with surface imperfections
- ISO 8501-4 – Initial surface conditions, preparation grades and flash rust grades in connection with water jetting
It’s important to note that EN BS ISO 8501-1 and EN BS ISO 8501-2 are currently under development and will soon be updated. As of this article’s publication in August 2026, there are no set release dates for the updated standards.
We primarily use ISO 8501-1 and -2 assessments in our surface preparation treatments, as the majority of our projects see us removing, preparing, and re-coating steel surfaces entirely or in localised areas.
There are four rust grades in ISO 8501-1 which help teams identify the steel’s surface condition, some of which apply to surfaces long exposed once old coatings have failed:
A – The majority of the surface is covered in mill scale (a layer of iron oxide that forms on hot-rolled steel that offers temporary corrosion protection) with little to no rust.
B – The mill scale has begun to flake, and the surface has begun to rust.
C – The mill scale has completely flaked away or can be scraped away, and the surface has rusted with very slight pitting visible to the naked eye
D – The surface is completely covered with rust and pitting visible to the naked eye
ISO 8501-2 does not define rust grades, and instead focuses solely on preparation grades.
Surface preparation grades and methods
Every method of surface preparation comes with surface preparation grades as outlined in ISO 8501. These grades are visual standards for the condition and cleanliness of the steel’s surface before coating application, once old coatings have been removed entirely or locally. The aim is to clean until you reach the desired grade.
Both ISO 8501-1 and -2 have their own surface preparation grades, but they are very similar. The main differences are:
- 8501-1 has an extra blast cleaning grade
- 8501-2 grades cover removing old paint coatings
- 8501-2 grades have a ‘P’ prefix before each, which indicates partial or localised cleaning
Let’s look at these surface preparation grades and the cleaning methods they’re linked to.
Manual and mechanical cleaning: Grades ST2 and ST3, and P ST2 and P ST3
Manual and mechanical cleaning methods cover preparing the steel with hand or power tools. The tools vary depending on the grade of preparation a team is looking to achieve.
- ST2 / P ST2 – This grade is defined as thorough hand and power tool cleaning. Achieving this grade means removing loose rust, loose mill scale, loose paint, and other contaminants. Typical tools used are wire brushes, scrapers, and mechanical grinders. Aiming for this grade instead of ST3 or P ST3 can be due to time and money constraints, job size, and environmental factors such as tight spaces, safety restraints, and a non-corrosive atmosphere.
- ST3 / P ST3 -This grade is defined as very thorough hand and power tool cleaning. Achieving this grade means removing loose rust, loose mill scale, loose paint, and other contaminants like ST2, but with more intensity to leave behind a metallic sheen on the surface. Typical tools used are more heavy-duty mechanical grinders, rotary wire brushes, sanding machines, and needle guns.
Abrasive blast cleaning: Grades SA1, SA2, SA2.5, and SA3, and P SA2, P SA2½, and P SA3
Abrasive cleaning methods cover preparing the steel with abrasive blasting. At Cladspray, we use grit blasting in our commercial blast cleaning services, which involves blasting a surface at speed with tiny, angular pieces of metal using a jet system to remove contaminants. Some jobs do not require steel to reach SA3 or P SA3 grade, typically because of factors such as asset lifespan, project requirements, and the corrosiveness of the environment.
- SA1 – This grade is defined as light blast cleaning. Achieving this grade means removing only loose rust, loose mill scale, and other contaminants, with potential residue and staining left behind.
- SA2 / P SA2 – This grade is defined as thorough blast cleaning. Achieving this grade means removing most rust, mill scale, paint, and other contaminants, with potential stains left behind.
- SA2½ / P SA2½ – This grade is defined as very thorough blast cleaning. Achieving this grade means removing nearly all rust, mill scale, paint, and other contaminants with enough intensity to leave the surface grey.
- SA3 / P SA3 – This grade is defined asblast cleaning to visually clean steel. Achieving this grade means removing all rust, mill scale, paint, and other contaminants with enough intensity to leave the surface metallic. This is the standard that Cladspray teams strive to achieve when delivering our surface preparation services on steel cladding and steelwork.
Expert steel surface preparation services from Cladspray
Cladspray has been performing surface preparation for over a decade, with our team members bringing years of past experience to offer services backed by significant expertise and authority. When your cladding is looking shabby, or your steelwork requires a full coating reset, we utilise our experience in surface preparation to assess exactly what needs to be done.
We bring the right tools and work to the highest standards, ensuring our work is compliant with the ISO 8501 series standards for the best results possible.
If you’re interested in our surface preparation services, or you need the full package for a building refresh, get in touch with our knowledgeable team today.
