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Are All Surface Prep Standards Created Equal? Part 1


By Lee Wilson

ISO8501-1 Is recognised as the most widely specified international surface preparation standard outside of the United States. However, the standard is subject to increasing controversy with regards to its technical content, technical descriptions and what some call ambiguity and confusion.

Blasting metal
© / Glenj

The standard ISO8501 is the international standard for dry abrasive blasting. The standard was derived from the Swedish Standard SIS0055900 in 1988.

“Can this be true?” I hear you say. Well, unfortunately, it is, and the issues have been raised on many occasions. I raised this question in JPCL back in 2011, and took the debate up with ISO directly. At the time, a TG Technical Group was in place revising the standard. However, there were no changes made at that point, nor have there been since the latest revision, back in 2007.

A Bit of History

The standard ISO8501 is the international standard for dry abrasive blasting. The standard was derived from the Swedish Standard SIS0055900 in 1988. This standard was the first one developed for surface preparation. It depicted four rust grades and four blast standards.

SIS0055900, developed by the Swedish Corrosion Institute, showed four blasting standards: Sa3, Sa2½, Sa2, and Sa1. “Why not 1,2,3 and 4?” I hear you ask. Well, the Institute had Sa1, Sa2, and Sa3, then later found an in-between standard that was better than Sa2, but not as good as Sa3. So they called it Sa2½ to avoid confusion with the three standards already published.

Thus, even at this stage, standards organizations were trying (I believe, however, unsuccessfully) to simplify it all. The Swedish standard was later superseded by International Standard ISO8501-1, which is a slight extension.

And this, my learned friends, is where the Sa standards were originally derived from. So what does Sa actually stand for? I have heard many opinions in the past: For example, Standard Achieved, Swedish Approved, etc., etc. However, plain and simple, it stands for “sand,” as that was the abrasive medium used to blast the 4 rust grade panels. I know, as I contacted the Swedish Institute many years ago for clarification on the subject.

Short Standard

So what is causing the problems, and where does the ambiguity arise?

ISO 8501-1 Standard Sa2½ ,”Very Thorough Blast Cleaning,” states:

“When viewed without magnification, the surface shall be free from visible oil, grease and dirt, and from mill scale, rust, paint coatings and foreign matter. Any remaining traces of contamination shall show only as slight stains in the form of spots or stripes.”

That’s all, folks—just a sentence with an enigmatic phrase: “… slight stains in the form of spots or stripes.” Can someone please tell me what that’s all about? The debates over the above definition of this standard have been ongoing for decades, as you are well aware: The contractor, the client, and the manufacturer all have different interpretations of “slight.”

Tank blasting
© / Glenj

A short, vague standard opens the door to confusion and disputes.

What are these “slight” stains?

I have had all the usual answers to this question about stains, such as, “they’re mill scale”; “they’re rust”; “they’re old coating.” But my answer is always the same. The standard states that the blast-cleaned surface shall be free visually of all of the above-mentioned contamination. If a stain is visible, then it is in contradiction of the standard. And that’s exactly what the standard does: It contradicts itself and doesn’t offer a solution or a get-out-of-jail-free card. The standard simply causes confusion for inspectors and supervisors unlucky enough to fall into a debate over the definition of “slight” and of “slight stains.”

I chose Sa2½ as an example for the simple reason that it is the most widely used dry abrasive blasting standard in our industry, yet many now believe that it is seriously in need of a revamp … but then again, is it?

I don’t think Sa2½ needs a revamp, because it would only confuse the already confused, including me. However, a quantifiable measured result should be included. I do believe that we have salvation in the shape of the joint SSPC/NACE standards.

More to Come

In the next installment of this series, I’ll look at SSPC-SP 10/NACE No. 2, “Near-White Metal,” the SSPC/NACE standard most equivalent to ISO8501-1 Standard Sa2½. Then we can get to the bottom of what really makes a standard—and whether the ISO and SSPC/NACE surface preparation standards that we think of as equivalent are really that equivalent at all.


Lee Wilson

Lee Wilson, CEng, FICorr, is a NACE Level 3-certified CIP Instructor, NACE Corrosion Specialist, NACE Protective Coating Specialist and Senior Corrosion Technologist, as well as an ICorr Level 3 Painting Inspector and Level 2 Insulation Inspector. The author of the best-selling Paint Inspector’s Field Guide, Lee was named one of JPCL Top Thinkers: The Clive Hare Honors in 2012. Contact Lee.



Tagged categories: EMEA (Europe, Middle East and Africa); Engineers; Inspection; Institute of Corrosion (ICorr); Lee Wilson, CEng, MICorr; NACE; North America; Quality Control; Quality control; SSPC; Asia Pacific; Latin America; Surface Preparation

Comment from BANGYIH CHEN, (1/19/2017, 2:40 AM)

Agree your statement.

Comment from David Shaw, (1/19/2017, 5:20 PM)

We've all seen very thorough blasted steelwork, but does not have a uniform appearance, due to nozzle angle or overworking or type of abrasive or original rust grade. I would like to see this condition with a better description. Interesting article, look forward to the follow up.

Comment from David Hlosta, (1/27/2017, 3:25 AM)

Very interesting and informative article. We also have the same problems with surface cleanliness evaluation in the Czech Republic :-)

Comment from Lee Wilson, (1/31/2017, 9:46 AM)

The ISO standard has all the technical potential to be a great standard every inspector globally has at one time or another came across the Sa standards. ISO just need to look at implementing a quantifiable and measurable figure to which the standard can be compared against in regards to acceptance and rejection criteria. That is why in my opinion the ISO standard can not be classed as an equivalent to the joint NACE/SSPC surface preparation standards.

Comment from OM PRAKASH JAT, (2/14/2017, 6:34 AM)

Its very informative but the definition of stains or contents of stains is not yet cleared..

Comment from Lydia Frenzel, (2/15/2017, 3:07 AM)

A good reference is by Rob Francis "Surfaces, Standards and Semantics: A close Look at Surface Cleaning Standards" JPCL May 2015, p. 24

Comment from Lee Wilson, (2/16/2017, 12:29 PM)

Yes Lydia a good reference which picks up from my original article in JPCL March 2011

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