Abrasive Blast Cleaning Grades

Quick Answer: SSPC defines four abrasive blast cleanliness grades by the percentage of staining permitted on the blasted surface: SP 5 White Metal allows 0% staining (100% clean), SP 10 Near-White allows 5% staining (95% clean), SP 6 Commercial allows 33% staining, and SP 7 Brush-Off removes only loose material and leaves tight mill scale intact. The required grade is dictated by the coating product data sheet and the service environment.

SSPC-SP 5 White Metal Blast (NACE No. 1)

SSPC-SP 5, "White Metal Blast," is the highest abrasive blast cleanliness standard. The surface is free of all visible oil, grease, dust, dirt, mill scale, rust, coating, and other foreign matter, with 0% staining allowed. Under SSPC-VIS 1 photographs, a white-metal surface appears as a uniform gray-white metallic color with no discoloration. SP 5 is specified for the most demanding service: immersion, high-humidity environments, chemical exposure, and high-performance coating systems such as inorganic zinc silicate primers, which require a clean, bare-steel substrate for galvanic protection to function. Achieving true SP 5 is time-consuming and expensive, so specifications reserve it for services where anything less would risk premature failure.

SSPC-SP 10 Near-White (NACE No. 2)

SSPC-SP 10, "Near-White Blast," permits 5% staining — light discoloration randomly distributed over no more than 5% of each unit area of surface. The surface is 95% clean and free of all visible contaminants except for the allowed staining. SP 10 is specified for high-humidity, chemical, and immersion service where SP 5 is not required but where a high-quality, nearly white surface is still needed. Many specifications accept SP 10 for the same coating systems that would otherwise require SP 5, because the cost and time difference is significant and the 5% staining allowance rarely affects coating performance.

SSPC-SP 6 Commercial Blast (NACE No. 3)

SSPC-SP 6, "Commercial Blast," allows 33% staining. Two-thirds of the surface is free of all visible contaminants, and the remaining one-third may show light staining, discoloration, or very tight residues. SP 6 is the most common blast grade for atmospheric service — structural steel, bridges, tank exteriors — where the coating system is not exposed to immersion or aggressive chemicals. It provides a sufficient profile and cleanliness for most epoxy primer and polyurethane topcoat systems.

SSPC-SP 7 Brush-Off Blast (NACE No. 4)

SSPC-SP 7, "Brush-Off Blast," is the lightest abrasive blast grade. It removes loose rust, loose mill scale, loose paint, and other loose foreign matter but leaves tight, adherent mill scale intact. SP 7 produces a profile only where the abrasive impacts bare steel; intact mill scale areas retain their original smooth surface. Brush-off is specified for maintenance painting where the existing coating is largely sound and only loose material needs removal before recoating.

Blast Grade Staining Comparison

When Each Grade Is Specified

The specification or coating product data sheet dictates the required blast grade — the inspector verifies that the achieved surface matches the specified standard. Selection follows the service environment and coating chemistry:

• Immersion or chemical service typically requires SP 5 White Metal. Inorganic zinc silicate primers, which rely on direct steel contact for galvanic protection, almost always require SP 5 or SP 10.
• High-humidity, marine, or moderate chemical service often specifies SP 10 Near-White as a cost-saving alternative to SP 5.
• Atmospheric structural steel such as bridges, buildings, and tank exteriors generally specifies SP 6 Commercial.
• Maintenance recoating of sound existing coatings often specifies SP 7 Brush-Off, since removing tight adherent coating is unnecessary and wasteful.

Reading Staining Correctly

The staining percentages are per unit area, not per total surface. SP 6's 33% allowance means that in any given inspection area, up to one-third may show light staining. An inspector who sees a unit area with 33% staining spread evenly is looking at a conforming SP 6 surface. The same level of staining on an SP 10 near-white job would be a non-conformance, because SP 10 permits only 5%. Staining refers to shadows, streaks, or slight discoloration remaining after blasting — visible rust, paint, or mill scale is a contaminant, not "staining."

Exam Traps

• SP 7 does not remove tight mill scale. A question that says "brush-off blast removes all mill scale" is false — SP 7 removes only loose material.
• SP 10 is not the same as SP 5. The 5% staining allowance is the key difference. If a specification calls for SP 5 and the contractor delivers SP 10, that is a non-conformance even though the surfaces look similar.
• NACE numbers are not in ascending order of cleanliness. NACE No. 1 = SP 5 (cleanest), NACE No. 2 = SP 10, NACE No. 3 = SP 6, NACE No. 4 = SP 7 (least clean). The NACE numbers were assigned chronologically, not by cleanliness rank — a favorite exam trap.