SSPC-SP10 surface preparation (NACE No. 2): the near-white metal blast guide

What is SSPC-SP10 surface preparation?

SSPC-SP 10/NACE No. 2 is the near-white metal blast cleaning standard. Viewed without magnification, the steel must be free of all visible oil, grease, dust, dirt, mill scale, rust, coating, oxides and corrosion products, and random staining is limited to 5 % of each unit area of 9 in² (3 in × 3 in).

That staining may only be light shadows, slight streaks or minor discolorations caused by stains of rust, mill scale or previously applied coating. A fleck of tight mill scale, rust in the bottom of a pit or a spot of old primer is a residue, not a stain, and it fails the grade.

Four details are often missed on site:

• The unit area is the unit of judgement. The 5 % is not averaged over a whole shell: each 9 in² square (about 76 mm × 76 mm) must meet it on its own.
• The grade applies immediately before coating. Rust-back between blasting and priming means the surface is no longer SP 10.
• Oil and grease come off first, by solvent cleaning (SSPC-SP 1). Blasting over oil spreads it.
• SP 10 does not define the surface profile. Cleanliness and roughness are separate requirements.

The written definition governs. The SSPC-VIS 1 photographs are a visual aid for agreeing on the expected appearance, not a substitute for the text.

SSPC-SP10 vs SP5, SP6 and the other SSPC surface preparation standards

SP 10 belongs to a family of cleanliness standards. Those you will meet in North American specifications, with their NACE pair and the closest ISO 8501-1 grade:

These ISO equivalences are approximate, not exact. ISO 8501-1 judges cleanliness against reference photographs for each initial rust grade (A to D); SSPC judges against written criteria and a staining limit. An inspector working to one system can reject a surface the other accepts. Our comparison of ISO 8501 vs SSPC and which one governs shows how to settle this in a specification.

Note that SP 2 and SP 3 are tool cleaning methods that leave adherent material in place, while SP 6 is an abrasive blast. None of them substitutes for SP 10 when a data sheet asks for near-white metal.

NACE No. 2, SSPC-SP 10 and AMPP: one standard, three names

"NACE No. 2" and "SSPC-SP 10" are the same joint standard, published by both associations under both designations. In 2021 NACE International and SSPC merged to form AMPP (Association for Materials Protection and Performance), which now maintains it. The requirement did not change; only the publisher did. Cite the designation and edition your specification invokes, and state the equivalence once. Our NACE, SSPC and AMPP designation map lists the other joint standards.

Anchor profile: what SP 10 does not specify

Blasting also cuts a roughness into the steel: the anchor profile. SP 10 says nothing about its depth. The target range comes from the coating manufacturer's product data sheet and the project specification, because it depends on the product and its film build. Too shallow limits adhesion; too deep leaves peaks the first coat may not cover.

• ASTM D4417 Method C: replica tape, read with a spring micrometer. It gives a value and a physical sample for the QA file. Methods A (comparator) and B (depth micrometer) are the alternatives.
• ISO 8503: comparators for grit and shot profiles (ISO 8503-1) and the replica tape method (ISO 8503-5).

The number and location of readings belong in the inspection and test plan; SSPC-PA 17 is often cited for this.

Abrasives for near-white metal blasting

The abrasive decides profile shape, productivity and the waste stream. Choose it against the specified profile, the substrate and the site, not by habit.

• Steel grit: angular and recyclable, suited to shops and enclosed work with recovery.
• Steel shot: round, peened profile; usually blended with grit when a coating needs an angular profile.
• Expendable mineral and slag abrasives: common in open field work; cleanliness covered by SSPC-AB 1.
• Garnet: natural, hard, relatively low-dust.
• Aluminum oxide: hard and recyclable; a common choice on stainless steel and non-ferrous metals to avoid iron contamination.
• Silica sand: heavily restricted because of respirable crystalline silica; check provincial health and safety rules first.

The abrasive must itself be clean (SSPC-AB 2 for recycled ferrous abrasive; ASTM D4940 for conductivity of soluble contaminants), and so must the compressed air (blotter test, ASTM D4285).

Invisible contaminants: soluble salts and dust

A surface can meet SP 10 visually and still fail the coating, because the two contaminants that most often cause premature failure are not visible.

Soluble salts. Chlorides and sulfates concentrate in pits and survive dry blasting. Under an immersion coating they drive osmotic blistering and under-film corrosion. They are extracted with the Bresle patch method (ISO 8502-6) and measured by conductivity (ISO 8502-9). The acceptance threshold is set by the specification or the coating manufacturer, not by SP 10. If the result is too high, the usual remedy is washing with clean water, re-blasting and re-testing.

Dust. Fines left on the surface form a weak layer under the primer. ISO 8502-3 rates dust with a tape test, by quantity and particle size; the specification sets the acceptable rating.

Ambient conditions: dew point and surface temperature

Condensation on blasted steel is invisible and starts rust-back within the shift. The common rule, used in ISO 8502-4 guidance and most specifications, keeps the steel at least 3 °C above the dew point during final blasting and application. Humidity and minimum temperature limits come from the specification and the data sheet.

Measure steel temperature with a contact gauge, not air temperature, and record conditions throughout the job. In Quebec and Ontario, a winter shutdown usually means heating and dehumidification; see our guide to cold-weather epoxy coating application during a winter shutdown.

Time between blasting and primer: flash rust

SP 10 sets a condition, not a number of hours: the surface must still meet near-white metal when the primer goes on. Flash rust or recontamination means re-blasting. If your specification or data sheet sets a maximum delay, that value governs. In practice, blast only what can be primed while conditions hold, and re-inspect steel blasted the previous shift. Wet abrasive blasting and waterjetting standards define their own flash-rust grades, and the data sheet states which one the primer tolerates.

When SP 10 is required vs SP 6 vs SP 5

The product data sheet usually states the minimum preparation for each service. The general pattern:

SP 5 costs more blasting time than SP 10 for a narrow gain in cleanliness; specify it when the manufacturer or owner requires it, not by default. The ISO 12944 corrosivity categories help frame the exposure, and our article on tank lining within a shutdown window shows where blasting sits in the schedule.

SSPC-SP10 inspection checklist

1. Before blasting: oil and grease removed (SP 1); welds and edges dressed as specified (ISO 8501-3 grades P1 to P3); initial rust condition recorded.
2. Equipment: air blotter test (ASTM D4285); abrasive type and cleanliness documented.
3. Ambient: temperature, humidity, dew point and steel temperature logged.
4. Cleanliness: written SP 10 criteria applied per unit area.
5. Profile: ASTM D4417 Method C or ISO 8503, against the data sheet range.
6. Salts: ISO 8502-6 and ISO 8502-9, against the specified threshold.
7. Dust: ISO 8502-3, against the specified rating.
8. Before primer: re-check for rust-back; re-blast if needed.
9. Records: readings, locations, times and batch numbers in the QA/QC file.

Common mistakes after SP 10 blasting

• Judging the whole surface instead of each unit area.
• Skipping salt tests because the steel "looks clean".
• Treating ISO and SSPC grades as interchangeable without naming the governing system.
• Blasting ahead of the primer crew and coating steel that has started to rust back.
• Using air temperature instead of steel temperature for the dew point margin.
• Specifying profile "as deep as possible" instead of the data sheet range.

FAQ

What is the difference between SSPC-SP10 and SSPC-SP6? Both are abrasive blast grades. SP 10 limits staining to 5 % of each unit area; SP 6 allows up to 33 %. SP 10 is the usual minimum for immersion and severe service.

Is SSPC-SP 10 the same as Sa 2½? They are the closest equivalents, not identical: ISO 8501-1 relies on photographs, SSPC on written criteria. State which one governs acceptance.

Does SSPC-SP 10 specify a surface profile? No. The range comes from the coating data sheet and the specification, and is measured separately, typically by replica tape (ASTM D4417 Method C).

Can SP 10 be achieved with power tools? No, it is an abrasive blast standard. Where blasting is impossible, SP 11 or SP 15 apply, and only if the coating system is qualified for them.

Is NACE No. 2 still valid after the AMPP merger? Yes. The joint standard is maintained by AMPP and is still cited under its legacy designations.

In practice

Induscoat prepares and coats industrial equipment in Quebec and Ontario to the grade your specification invokes, whether written as SSPC-SP 10/NACE No. 2 or Sa 2½, with profile, salt, dust and ambient records in the QA/QC file. Our protective coatings service covers preparation and application in the shop and on site, and our corrosion protection page describes the equipment we treat. If a specification leaves the governing system, the profile or the salt threshold undefined, we raise it before pricing. Send the specification and equipment details to request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.