Surface Prep Standards - A Quick Summary

"Never paint a dirty surface” is something of a mantra in industrial painting circles. Without proper surface preparation, a coating will not adhere properly to the substrate and the system will fail. Existing standards — published and continuously revised by independent industry oversight agencies — do a good job of standardizing surface prep outcomes, but naming conventions between agencies can make the standards confusing.

The most commonly accepted standards for contractors and organizations are the NACE International/Society for Protective Coatings (SSPC) Joint Surface Preparation Standards. (NACE and SSPC have recently merged to form AAMP, The Association for Materials Protection and Performance.) While these documents standardize surface prep results and provide a template to meet job specifications, they can be confusing because the numbers, given for each level of surface preparation, don’t always correlate; higher numbers do not necessarily mean a higher degree of surface prep, as one might expect.

To make things a little clearer, below is a list of brief explanations of each standard with a visual description.

SSPC-Surface Preparation (SP)-1: Solvent Cleaning This method of surface preparation is meant to remove soluble substances from steel. Before a paint or other protective coating is applied, a solvent is used to remove all visible oil, grease, dirt, drawing or cutting compounds, or other soluble contaminants. Solvents may include steam, emulsifying agents, or other cleaning compounds.

SSPC-SP-2: Hand Tool Cleaning Hand tool cleaning refers to surface preparation that uses non-power handheld tools to clean a steel surface. Hand tool cleaning is intended to remove all loose (as opposed to adherent) mill scale, rust, paint, and other contaminants that may be detrimental to a coating application. According to SSPC, “adherent” contaminants are those that can’t be removed by lifting them off with a dull putty knife.

SSPC-SP-3: Power Tool Cleaning As in hand tool cleaning, SP-3 is a method of steel surface preparation intended to remove all loose (as opposed to adherent) mill scale, rust, paint, and other contaminants that may be detrimental to a coating application. As the name suggests, SP-3 refers to using power tools to clean the surface. Again, according to SSPC, adherent contaminants are those that can’t be removed by lifting them off with a dull putty knife.

NACE No. 1/SSPC-SP-5: White Metal Blast Cleaning This SSPC/NACE joint standard describes the cleaning of a steel surface, previously painted or unpainted, to a white metal condition through the use of abrasive blast media. White metal is a term used to describe a surface that’s uniformly free of all foreign matter and is white or gray in appearance. According to SSPC, a surface blasted to white metal should, without magnification, be free of all visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter.

NACE No. 3/SSPC-SP-6: Commercial Blast Cleaning No. 3/SP-6 is another joint standard describing the cleaning of a steel surface through the use of abrasive blast media. It includes instructions for use prior to blast cleaning, as well as for the inspection of the cleaning after it has been conducted. Similar to a white metal blast cleaning, surfaces prepared to an No. 3/SP-6 standard should be, without magnification, free of all visible oil, grease, dust, dirt, mill scale, rust, coating, oxides, corrosion products, and other foreign matter. Random staining from previous exposure to those contaminants is acceptable; however, this is only so long as the contamination does not comprise more than 33 percent of each area “unit,” as described by the standard. Such staining may take the form of light shadows, slight streaks, or minor discolorations caused by stains of rust, mill scale, or previously applied coating, according to SSPC.

NACE No. 4/SSPC-SP-7: Brush-Off Blast Cleaning This joint standard conveys the requirements for cleaning a steel surface, painted or unpainted, with the use of abrasive blast media. It contains descriptions of the required end condition of a surface that has undergone brush-off cleaning, as well as the necessary methods for verifying the asset’s end condition. All oil, grease, dirt, and dust must be cleared from the surface when viewed without magnification. Loose mill scale, rust, and coatings must also be removed according to this standard, but tightly adherent mill scale, rust, and coatings may remain. These are considered tightly adhered if they cannot be removed by lifting them with a dull putty knife.

NACE No. 2/SSPC-SP-10: Near-White Metal Blast Cleaning This standard conveys the requirements for cleaning a steel surface, painted or unpainted, with the use of abrasive blast media. It also includes instructions for achieving and verifying the standard’s required end condition. As with a commercial blast, the prepared surface must be free, when viewed without magnification, of visible oil, dust, dirt, grease, mill scale, rust, coating, oxides, corrosion, and other foreign matter, except for a limited amount of acceptable staining. Unlike with a commercial blast, only five percent of each area unit, as defined by the standard, may exhibit staining. This five percent may consist of light shadows, slight streaks minor discolorations that could be the result of exposure to rust, mill scale, or of a previous coating.

SSPC-SP-11: Power Tool Cleaning to Bare Metal  This standard describes the requirements for taking a surface to bare metal, while ensuring a minimum surface profile of 1 mil (25.4 microns). It is used in situations where abrasive blasting is not possible or feasible. Unlike SP-3, this standard requires the creation or preservation of a surface profile. Unlike SP-15, this standard does not allow for stains from mill scale, rust, or paint to remain on the surface.

NACE No. 8/SSPC-SP-14: Industrial Blast Cleaning This standard conveys the requirements for cleaning a steel surface, painted or unpainted, with the use of abrasive blast media. It also includes instructions for achieving and verifying the standard’s required end condition. As with a commercial blast and a near-white commercial blast standards, the prepared surface must be free, when viewed without magnification, of visible oil, dust, dirt, grease, mill scale, rust, coating, oxides, corrosion, and other foreign matter, except for a limited amount of acceptable staining. No. 8/SP-14 differs from a commercial blast and a near-white commercial blast in regards to the acceptable area for residue and surface stains remaining. This standard allows for tightly adhering mill scale, rust, and coatings, as well as surface stains to remain on 10 percent of each unit area, as described by the standard. Surface stains may consist of light shadows, slight streaks, and minor discolorations that could be the result of exposure to rust, mill scale, or of a previous coating.

SSPC-SP-15: Commercial Grade Power Tool Cleaning Like SP-11, this standard describes the requirements for taking a surface to bare metal, while ensuring a minimum surface profile of 1 mil (25.4 microns). Unlike SP-11, SP-15 allows for random staining to persist on the substrate.

SSPC-SP-16: Brush-Off Blast Cleaning of Non-Ferrous Metals This standard governs surface preparation for non-ferrous metals before the application of a protective coating. It is used when adding a surface profile to stainless steel, galvanized steel, copper, and other metals that are not carbon steel. It requires a minimum surface profile of .75 mil (19.0 microns) and for the surface to be free of loose coating and other contaminants, as verified by a visual inspection.

Every coating contractor needs to prepare the substrate appropriately, whether the job is to coat a highway bridge, line a steel tank, or protect a brewery floor. It’s helpful, therefore, to know how to use these standards appropriately.

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Source: This article was adapted from an an article that originally appeared in CoatingsPro Magazine.

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