cleaning standards used in NorthAmerica are developed jointly bySSPC: The Society for ProtectiveCoatings and NACE International.Each of the five levels of cleanlinessis defined in a separate written stan-dard. A set of visual reference pho-tographs (SSPC-VIS 1) has beendeveloped as a supplement to (butnot a substitute for) the five writtenstandards. Most other countries usethe standard produced by ISO (ISO8501-1), originally developed inSweden (where it was SwedishStandard SIS 05 59 00) and stilloften referred to as “the SwedishStandard.” This standard containswritten descriptions, as well as pic-tures, of abrasive-blasted and hand-and power-tool-prepared surfaces ina single book.

ence visual cleanliness standards aremill scale, rust and old paint, which maybe adherent or loosely bound to the sur-face. The increasing standards requiremore of this contamination to beremoved. Loose material will beremoved first, leaving adherent contami-nation. Further treatment will remove thismaterial but still leave traces of adherentcontamination known as “stains.” Onlyafter very thorough blasting will thesestains be removed to leave a completelyclean surface with no contamination visi-ble to the naked eye. These stages areshown in Figure 1, with contaminationreferred to as “scale” whether it is millscale, rust or old paint.

Standards for Cleaning SteelThere are two main providers of sur-face preparation standards. Blast-

oor surface preparation isoften considered the maincause for premature coat-ing breakdown. Visualcleanliness of a steel sur-

face is perhaps the most importantrequirement influencing coating durabili-ty. However, there are arguments, con-fusion and disagreements regardingboth cleanliness levels and correlationbetween the various industry standards.Furthermore, new methods of power-tool cleaning are being introduced,which claim to provide cleanliness lev-els approaching those of the higherclasses of abrasive blast-cleaning. Ofcourse, visual cleanliness is not theonly surface preparation parameterinfluencing coating durability, with sur-face profile, removal of oil and grease,treatment of soluble salts and otherrequirements also being important.

This article will discuss visual cleanli-ness for blast-cleaning, with a view onclarifying requirements for the specifier,contractor and inspector.

Removing Rust and ScaleIt is important to understand the type ofcontamination on a steel surface andhow it is removed during surface prepa-ration. The three contaminants that influ-

BasicTraining

Surfaces, Standards andSemantics: A Close Look at Visual Surface Cleaning Standards

PBy Rob Francis, R A Francis Consulting Services Pty Ltd

paintsquare.com / JPCL February 2015 27

Fig. 1: This figure displays the order in which contamination is removed from a surface, starting withloose material, then adherent contamination and then, finally, staining to leave a clean, profiled surface.Figures courtesy of the author unless otherwise specified.

28 JPCL February 2015 / paintsquare.com

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There are differences regarding therelationship between the visual stan-dards and the written standards. Withthe ISO standards, the visual stan-dards are an integral part of the stan-dard; that is, the reference to theapplicable pictures is part of the writ-ten description. The visual standardSSPC-VIS 1 can be used to supple-ment the written SSPC/NACE jointstandard definitions for abrasive blast-ing, but the written standard takesprecedence over the visual standardsin any dispute.

Relationship BetweenSSPC/NACE and ISO StandardsHighest Standard of CleaningIn the SSPC/NACE joint standard, thehighest standard of blast-cleaning isSSPC-SP 5/NACE No. 1, designated as

“White Metal” and defined as follows:“A white metal blast cleaned surface,

when viewed without magnification,shall be free of all visible oil, grease,dust, dirt, mill scale, rust, coating,oxides, corrosion products, and otherforeign matter.”

The highest standard of cleanlinessin ISO 8501-1 is Sa 3, designated as“visually clean steel” and defined as fol-lows:

“When viewed without magnification,the surface shall be free from visibleoil, grease and dirt, and shall be freefrom mill scale, rust, paint, coatingsand foreign matter. It shall then have auniform metallic colour and correspondto the prints designated Sa 3 in ISO8501-1.”

Both standards require the surface tobe completely clean with no visible con-

tamination when viewed without magnifi-cation. There is little doubt that the twostandards are identical in intent, and itwould be impossible to produce a blast-cleaned surface that met one standardbut not the other.

Second-Highest StandardThe joint standard for the next-bestclass is SSPC SP-10/NACE No. 2,“Near-White,” and is defined as follows:

“A near-white metal blast cleaned sur-face, when viewed without magnifica-tion, shall be free of all visible oil,grease, dust, dirt, mill scale, rust, coat-ing, oxides, corrosion products, andother foreign matter. Random stainingshall be limited to no more than 5 per-cent of each unit area of surface ...and may consist of light shadows,slight streaks, or minor discolorations

paintsquare.com / JPCL February 2015 29

caused by stains of rust, stains of millscale, or stains of previously appliedcoating.”

ISO's second-highest class of clean-ing is Sa 21⁄2, or “very thorough” blast-cleaning, defined as follows:

“When viewed without magnification,the surface shall be free from visibleoil, grease and dirt, and shall be freefrom mill scale, rust, paint, coatingsand foreign matter. Any remainingtraces of contamination shall show onlyas slight stains in the form of spots orstripes.”

Both definitions require a very cleansurface with no rust, mill scale, oldcoating or other matter. However, thereare two differences. First, both allow“staining” but define it slightly different-ly, although not clearly. The 2007 revi-sion of ISO 8501-1 Sa 21⁄2 implies thatstaining is “traces of contamination.”Although “staining” is not defined in the2007 joint standards, SSPC’sProtective Coatings Glossary definesstaining as "An area of a surface which,when compared to adjacent areas, hasan equal surface profile but is discol-ored (usually darker) with a materialhaving no apparent volume. The materi-al cannot be removed by methods com-monly used to remove dust, but can beremoved by more thorough abrasiveblasting when abrasive blasting is used,or more thorough power tool cleaningwhen power tool cleaning is used.”

It would appear that both standardshave the same requirement in thisrespect — namely, that the surfaceshall be completely clean, but traces ofdiscoloration are allowed.

The second difference is that thejoint standard actually quantifies thislevel of staining to no more than 5 per-cent, whereas the ISO standard usesthe word “slight.”

Third-Highest Standard of CleaningThe next level of cleanliness in the jointstandards is SSPC-SP 6/NACE No. 3,or a “Commercial” blast-cleaned sur-face. This is defined the same as“Near-White” except that the amount ofpermissible staining for a “Commercial”

finish is increased to 33 percent of thesurface.

“A commercial blast cleaned surface,when viewed without magnification,shall be free of all visible oil, grease,dust, dirt, mill scale, rust, coating,oxides, corrosion products, and other

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the mill scale, rust, paint, coatings andforeign matter. Any residual contamina-tion shall be firmly adhering.”

Sa 2 requires only that most millscale, rust and old paint be removed,and says nothing about whether stain-ing is allowed on cleaned areas.Contrary to the popular view that“Commercial” is equivalent to Sa 2, thejoint standard is clearly a bettermethod of cleaning as it requires allmill scale, rust and old paint to beremoved, whether adherent or not, with“white metal” over two-thirds of eachunit area of surface and staining overone-third. Comparing the two pho-tographs in Figure 2, although the“Commercial” grade appears to have agreater percentage of “white metal,”the difference in appearance betweenthe remaining contamination, whether

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foreign matter. Random staining shallbe limited to no more than 33 percentof each unit area of surface ... and mayconsist of light shadows, slight streaks,or minor discolorations caused bystains of rust, stains of mill scale, orstains of previously applied coating.”

The next level of cleaning accordingto ISO is Sa 2, or “thorough” blast-cleaning. The important visual aspectsof this definition are as follows:

“When viewed without magnification,the surface shall be free from visibleoil, grease and dirt, and from most of

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Fig. 2 The photo on the left shows a "Commercial" blast-cleaned surface (SSPC-SP 6/NACE No. 3),while the photo on the right shows a surface prepared to ISO Sa 2. While the two photos may appearsimilar, the joint standard is actually a higher degree of cleaning.Photos courtesy of SSPC and ISO.

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32 JPCL February 2015 / paintsquare.com

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staining or adherent, is not readilyapparent.

In 1999, a joint standard was intro-duced with a lower level of cleanlinessthan “Commercial,” known as SSPC-SP14/NACE No. 8, or an “Industrial” blast-cleaned surface. This allows evenly dis-tributed traces of tightly adherent millscale, rust and coating residues toremain on 10 percent of each unit areaof the surface. Staining may be presenton the remainder of the surface.

Unlike “Commercial,” the “Industrial”class does allow some tightly adherentcontamination on the surface, similar toISO Sa 2. The definition of “tightlyadherent” is the same for both stan-dards; as with the ISO standard, conta-mination is considered to be tightlyadherent if it cannot be lifted with a dullputty knife. The joint “Industrial” classallows stains over the remainder of thesurface (i.e. area that does not havetraces of contamination), whereas theISO class does not mention stains. The“Industrial” class quantifies the amountof adherent contamination that isallowed whereas the ISO standard onlyrequires that “most” of the adherentcontamination is removed. This wouldappear to be the only differencebetween the two; that is, whether

“most” can be quantified as 90 per-cent. Certainly, Sa 2 is very close to“Industrial,” but not as clean as“Commercial.” In addition, “Industrial”blast-cleaning exempts configurationssuch as back-to-back angles from the10 percent criteria and allows contami-nation to remain as long as an efforthas been made to remove it. ISO Sa 2does not discuss this issue.

Lowest Standard of CleaningThe lowest standard of cleaning in thejoint standards is SSPC-SP 7/NACE No.4, "Brush-Off” blast-cleaning. Thisrequires all loose material to beremoved but tightly adherent contamina-tion may remain. Similarly, ISO's lowestclass of cleaning, Sa 1 or “light” blastcleaning, simply requires the surface tobe free from poorly adhering mill scale,rust, paint, coatings and foreign matter.In both descriptions, all loose materialmust be removed, but tightly adherentmaterial may remain over the entire sur-face. Adhesion is defined similarly forboth. The joint standard has additionalinformation regarding the appearance,but the two descriptions are almost iden-tical. It would be impossible to find asurface that met one class without meet-ing the other.

Quantitative DescriptionsOne important feature of the joint stan-dard definitions is that they provide anumerical guide to acceptable levels ofcontamination. ISO provides qualitativedescriptions such as “slight” and“most” regarding areas contaminatedor cleaned.

Applying numbers to a quality levelwould generally be expected to providea clearer, more concise and lessambiguous method of describing sur-face cleanliness. However, it does notassist. If a number is asked for as aquality parameter in a standard or speci-fication, there normally has to be somemeans of measuring the parameter inquestion. In the case of surface cleanli-ness, the surface is judged visually. Thatis, the level of contamination is not, andpractically cannot be, measured but onlyestimated. Despite the quantitativerequirement, a qualitative assessmentmust be made. Therefore, the qualitativedescriptions given in the ISO standardsare arguably just as valid and, in manyways, preferable, as they implicitly rec-ognize that area estimation of contami-nation levels cannot be measured.

Summary of Blasting StandardsTable 1 provides a summary of theallowable levels of contamination forthe various standards of blast-cleaningshowing equivalence between the clean-liness levels. SSPC-SP 6, NACE No. 3,“Commercial” blast-cleaning is the onlystandard that clearly does not have anISO equivalent, although it is often con-sidered to be equivalent to ISO Sa 2.However, the table shows that“Commercial” is closer to ISO Sa 21⁄2than to ISO Sa 2. “Commercial” clean-ing has been specified for more exact-ing coatings such as inorganic zinc sili-cates, and the fact that it is a veryclean surface would explain why this

Cleanliness Class Loose Adherent StainingLevel Contamination

Highest SSPC-SP 5/NACE None None Nonestandard No. 1

ISO Sa 3 None None None

SSPC-SP 10/NACE None None <5% No. 2ISO Sa 2½ None None “Slight”

SSPC-SP 6/NACE None None <33%No. 3

SSPC-SP 14, NACE None <10% RemainderNo. 8 ISO Sa 2 None “Most removed” Not given

Lowest SSPC-SP 7/NACE None Allowed Not applicablestandard No. 4

ISO Sa 1 None Allowed Not applicable

Table 1: Permitted Levels of Contamination for Blasting Standards

discussed earlier, and the standardsachieved, are shown schematically inFigure 3.

While the joint and ISO standardsdisplay many similiarities, it isimportant to always adhere to thesurface preparation standard thatis written in the specification.

Rob Francis is aconsultant with over35 years of experi-ence in metals andmaterials, especiallyregarding protective

coatings. He is a JPCL contributingeditor and was named a JPCL TopThinker in 2012. He earned hisPh.D. in corrosion science from theCorrosion and Protection Center atUMIST in Manchester, U.K., and he isa NACE-certified Coating Inspector.JPCL

paintsquare.com / JPCL February 2015 33

has been successful.The process of blasting looking at

the stages of removal of contamination

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Fig. 3: This figure shows what contamination is allowed to remain and what needs to be removed foreach of the referenced blast-cleaning standards.