SSPC, NACE & ISO Surface Preparation Standards
A practical guide to SSPC-SP, NACE and ISO 8501-1 surface preparation grades, how they compare and what a complete prep specification needs.
Key takeaways
- SSPC and NACE (merged in 2021 as AMPP) publish written cleanliness standards. ISO 8501-1 defines grades through text and reference photographs. The two systems are similar but not identical.
- From most to least thorough, the blast grades are white metal (SP 5 ≈ Sa 3), near-white (SP 10 ≈ Sa 2½), commercial (SP 6 ≈ Sa 2), industrial (SP 14) and brush-off (SP 7 ≈ Sa 1).
- Equivalences between SSPC/NACE and ISO grades are approximate. Specify one system and use it consistently.
- Cleanliness grades say nothing about profile, dust or soluble salts. Each of those needs its own requirement.
- The written standard governs. Photographic guides such as SSPC-VIS 1 are visual aids, not the acceptance criterion.
A specification that just says “sandblast before painting” invites arguments. Surface preparation standards replace vague language with defined grades. Each grade states what must be removed, what may remain and how the result is judged. Owners, contractors and inspectors therefore work from the same yardstick.
This article summarizes the most widely used standards for steel, how the SSPC/NACE and ISO systems compare, and what a complete surface preparation requirement should include.
Who publishes the standards
- SSPC (originally the Steel Structures Painting Council, later SSPC: The Society for Protective Coatings) developed the “SP” series of surface preparation standards and the “VIS” visual guides.
- NACE International co-published several blast-cleaning standards with SSPC as joint documents, for example SSPC-SP 10/NACE No. 2.
- AMPP (Association for Materials Protection and Performance) was formed in 2021 by the merger of NACE and SSPC and now maintains these standards. The legacy designations remain in everyday use in specifications.
- ISO 8501 is the international series. ISO 8501-1 grades the cleanliness of uncoated steel and steel with all previous coatings removed. ISO 8501-2 covers partial preparation of previously coated steel. ISO 8501-3 covers welds, edges and other imperfections, and ISO 8501-4 covers water jetting.
Cleaning, hand-tool and power-tool standards
SSPC-SP 1, Solvent Cleaning, removes visible oil, grease, dirt and similar contaminants using solvents, detergents, steam or alkaline cleaners. It is a prerequisite for most other methods, because blasting and power tools spread oil instead of removing it.
SSPC-SP 2 (hand tool) and SSPC-SP 3 (power tool) remove loose rust, loose mill scale and loose coating. Tightly adherent material, meaning anything that cannot be lifted with a dull putty knife, may remain. Neither standard requires a profile.
SSPC-SP 15, Commercial Grade Power Tool Cleaning, and SSPC-SP 11, Power Tool Cleaning to Bare Metal, go further. Both require a profile of at least 1 mil (25 µm). SP 11 requires bare metal, with only slight residues allowed in the bottoms of pits on pitted steel. SP 15 permits random staining on up to 33% of each unit area. These standards are useful for spot repairs and for areas where abrasive blasting is not practical.
Abrasive blast cleaning grades
SSPC/NACE blast standards limit residue as a percentage of each unit area, defined as approximately 9 in² (58 cm²), roughly a 3 × 3 in square.
- White metal (SSPC-SP 5/NACE No. 1): no visible oil, grease, dust, mill scale, rust, coating, oxides, corrosion products or other foreign matter, and no staining.
- Near-white metal (SSPC-SP 10/NACE No. 2): as SP 5, except that random staining is allowed on no more than 5% of each unit area. This is the common grade for immersion linings and high-performance systems.
- Commercial (SSPC-SP 6/NACE No. 3): random staining is allowed on no more than 33% of each unit area. It is widely used for atmospheric service.
- Industrial (SSPC-SP 14/NACE No. 8): traces of tightly adherent mill scale, rust and coating are allowed on no more than 10% of each unit area, and staining is permitted. It is intended for situations where white or commercial blasting is not needed.
- Brush-off (SSPC-SP 7/NACE No. 4): removes loose material and leaves tightly adherent mill scale, rust and coating. The surface should still be roughened enough for adhesion.
- SSPC-SP 16: brush-off blast cleaning of galvanized steel, stainless steel and non-ferrous metals, with a minimum profile of 0.75 mil (19 µm).
SSPC, NACE and ISO equivalence table
The table below aligns the most common grades. Equivalences are approximate. SSPC/NACE standards use written criteria with percentage limits per unit area. ISO 8501-1 uses written descriptions and reference photographs with no percentage limits. For example, ISO Sa 2 permits some firmly adhering residue, which SSPC-SP 6 does not.
| SSPC | NACE (joint) | ISO 8501-1 (approx.) | Description | Residue allowed (SSPC/NACE) |
|---|---|---|---|---|
| SP 5 | NACE No. 1 | Sa 3 | White metal blast | None, no staining |
| SP 10 | NACE No. 2 | Sa 2½ | Near-white metal blast | Staining ≤ 5% of each unit area |
| SP 6 | NACE No. 3 | Sa 2 | Commercial blast | Staining ≤ 33% of each unit area |
| SP 14 | NACE No. 8 | No direct equivalent (between Sa 1 and Sa 2) | Industrial blast | Tightly adherent residue ≤ 10% of each unit area |
| SP 7 | NACE No. 4 | Sa 1 | Brush-off blast | Tightly adherent mill scale, rust, coating |
| SP 11 | — | No direct equivalent | Power tool to bare metal | Slight residue in pit bottoms only; ≥ 1 mil (25 µm) profile |
| SP 15 | — | No direct equivalent | Commercial-grade power tool | Staining ≤ 33%; ≥ 1 mil (25 µm) profile |
| SP 3 | — | St 3 | Power tool cleaning | Tightly adherent material |
| SP 2 | — | St 2 | Hand tool cleaning | Tightly adherent material |
| SP 1 | — | — | Solvent cleaning | Removes oil, grease and dirt only |
St 2 and St 3 are the closest ISO counterparts to SP 2 and SP 3, but the ISO and SSPC wording differs. On international projects, state which system governs instead of listing both as if they were interchangeable.
Water jetting, wet blasting and concrete standards
The joint SSPC-SP WJ-1 through WJ-4/NACE WJ-1 through WJ-4 standards replaced the earlier SSPC-SP 12/NACE No. 5. They define four degrees of cleaning by water jetting: WJ-1 (clean to bare substrate), WJ-2 (very thorough), WJ-3 (thorough) and WJ-4 (light). Flash rust after water jetting is graded light, moderate or heavy. ISO 8501-4 takes a parallel approach, with Wa 1, Wa 2 and Wa 2½ cleanliness grades and L, M and H flash-rust grades. Water jetting does not create a profile, so it relies on the profile that already exists.
For concrete, SSPC-SP 13/NACE No. 6 covers surface preparation of concrete for coating, and is commonly used together with ICRI 310.2R concrete surface profiles (see concrete surface preparation).
Initial rust grades and visual references
How a blasted surface looks depends on its starting condition. ISO 8501-1 and SSPC-VIS 1 both use four initial rust grades:
- A: steel largely covered with adherent mill scale and little or no rust.
- B: steel that has begun to rust, with mill scale starting to flake.
- C: mill scale rusted away or easily scraped off, with slight pitting.
- D: mill scale rusted away, with general pitting.
SSPC-VIS 1 (dry abrasive blast) also shows previously coated conditions and how different abrasives change appearance. Companion guides cover hand and power tool cleaning (SSPC-VIS 3), water jetting (SSPC-VIS 4/NACE VIS 7) and wet abrasive blasting (SSPC-VIS 5/NACE VIS 9). The photographs support the written standard; they do not replace it.
Writing a complete surface preparation specification
A cleanliness grade is only part of the requirement. A robust specification also covers:
- Pre-cleaning: SSPC-SP 1 before any mechanical preparation.
- Fabrication defects: edge and weld preparation grade, for example ISO 8501-3 P2 or P3.
- Profile: a range and a test method. See surface profile.
- Soluble salts: test method and maximum levels, especially for immersion service. See soluble salt contamination.
- Dust: for example, ISO 8502-3 tape test rating.
- Conditions and hold points: dew point limits, maximum time before priming and inspection sign-off.
Prepare a small reference area at the start of the job and get the owner, contractor and inspector to agree that it meets the specified grade. Photograph it and keep it as the visual benchmark for the rest of the work.
Frequently asked questions
Is SSPC-SP 10 the same as Sa 2½?
They are generally treated as equivalent, but they are not identical. SP 10 limits staining to 5% of each unit area; Sa 2½ allows only slight stains, defined by text and photographs. Specify the one that governs.
Which grade should I specify?
Follow the coating manufacturer’s product data sheet and the service environment. Immersion linings and zinc-rich primers usually call for SP 10 or SP 5. Many atmospheric systems accept SP 6.
Do NACE standards still exist after the merger?
Yes. Since 2021 AMPP maintains the former NACE and SSPC documents. Joint designations such as SSPC-SP 10/NACE No. 2 are still widely cited.
Does meeting a cleanliness grade guarantee a good surface?
No. A surface can meet SP 10 visually and still carry soluble salts or the wrong profile. Specify and test those separately.
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
