coating inspector forum

Let’s Talk Dull Putty Knives

KTA’s Certified Coating Inspector Forum

Issue No. 1 – May 2022

KTA’s Certified Coating Inspector Forum is designed to provide continuing education on standards, inspection practices, new instruments, and other topics to help keep certified AMPP and FROSIO coating inspectors current. It is occasionally written blog style, and represents the views of the author and KTA-Tator, Inc. It may or may not represent the views of AMPP: The Association for Materials Protection & Performance, even though SSPC, NACE, and AMPP standards are frequently referenced in the content.

Okay, now be honest… how many of you have a dull putty knife in your inspection instrument kit? And if you have one, are you sure it’s the correct dimensions and is “dull?”

Imagine the look on your client’s face as you attempt to explain to them that the acceptability of the cleanliness of a steel surface prior to application of a maintenance coating system is based, in part, on the use of a dull putty knife – ironically the same type of tool that may be used to perform the cleaning (think SSPC-SP 2, Hand Tool Cleaning). Curious looks aside, the dull putty knife is real, defined, and is referenced in surface preparation standards more than you may think. Let’s take a look.

First, what exactly is a “dull putty knife?” If I asked you to define what composes a dull putty knife, you may respond, “Well, I can’t define it, but I’ll know it when I see it.” The truth is it was well-defined in the 2011 version of the SSPC Glossary as:

2011 Version in SSPC Glossary – DULL PUTTY KNIFE (for use as an inspection tool): A commercially manufactured metal blade with these characteristics: width – 1-1/2 to 3-inches; length – 3 to 5-inches; thickness – 30 to 50 mils.The putty knife is acceptable for use if the thickness at end of the blade is not less than 25 mils or 75% of its original thickness, whichever is greater. It shall not be used if the edge is nicked or gouged, or if dry paint or other material is present along the edge that would prevent the blade from making intimate contact with the surface. When used to test paint, mill scale, or rust remaining on the surface after cleaning, the blade shall be held flat against the surface and at a maximum of 45 degrees to the surface. The corners of the blade shall not be used to dig at the residues.

A revised definition was established in 2018, as:

2018 Version in SSPC-SP2 and SP3 (but SSPC Glossary not updated – see “Uncovering a Conundrum” later in this article) DULL PUTTY KNIFE (for use as an inspection tool): A commercially manufactured, straight, flexible metal blade capable of returning to its original shape without permanent distortion after being bent by hand around a 28 to 33-cm (11 to 13-in) diameter mandrel (or pipe or other curved surface). The blade shall also have the following characteristics: length of approximately 75 to 125 mm (3 to 5 inches); thickness of approximately 760 to 1270 µm (30 to 50 mils); and a straight working edge approximately 40 to 75 mm (1.5 to 3 in) in width. The putty knife is acceptable for use if the thickness at the working edge of the blade is not less than 635 µm (25 mils) or 75% of its original thickness, whichever is greater*.  

NOTE: Some commercially manufactured, straight, flexible metal blades are between 500 and 760 µm (20 and 30 mils) in thickness. New blade thicknesses between 500 and 760 µm (20 and 30 mils) are permitted, provided the coating being tested is 20 mils or less in thickness, and the thickness of the blade is not worn to less than 20 mils.

* Exact dimensions of equipment manufactured using S.I. units may vary slightly from the S.I. values provided.

This revised definition (directly above) came about when SSPC-SP 2, Hand Tool Cleaning and SSPC-SP 3, Power Tool Cleaning were revised/updated in 2018. The revised definition is included in the Definitions section of both SSPC-SP2 and SP3. It was important that such a tool be “commercially available” and easily verifiable for conformance to the definition in the field. For example, the size of the mandrel to assess flexibility of the blade was based on the diameter of a 5-gallon paint pail, something that would be readily available on jobsites involving paint application. Determining the dimensions of the blade (width, length, and thickness), well that’s another story.

The SSPC Education Committee (that I chaired for nearly 12 years) was responsible for maintaining the SSPC Glossary. The subcommittee chair responsible for updating the SSPC-SP 2 and SSPC-SP 3 surface preparation standards at that time wanted to ensure that the dull putty knife (referenced in these standards as the inspection tool) was commercially available. So, I agreed to investigate, not knowing what I was really getting into. Equipped with my calibrated Ames spring micrometer and a tape measure, I headed off to a home improvement store (the Friday after Thanksgiving, better known as Black Friday) amongst the holiday shopping crowd to measure the length, width, and thickness of several metal, flexible blade putty knives to determine whether the “then proposed” dimensions of the blade were available. After several interruptions of “sir can I help you,” I discovered that indeed the originally proposed thickness was not commercially available. While the length and width complied, the thickness of the blade did not. Undoubtedly putty knife manufacturers had reduced the blade thickness to save on material costs with negligible, if any, impact on performance (putty knives are designed to apply putty and spackling, rather than serving as an inspection tool). It was discovered that a 30-50 mil thick blade was available across multiple suppliers, so the definition in the SSPC Glossary was updated to reflect what was commercially available, and SSPC-SP 2 and SP 3 were updated, balloted, and republished. Note however that an updated 2018 version of the SSPC Glossary was never published, which is problematic (see “Uncovering a Conundrum” below).

The use of visual guide SSPC-VIS 3 for the appearance of SSPC-SP 2 and SP 3 (hand and power tool cleaning), should be supplemented with the use of a dull putty knife, since the determination of loosely, versus tightly, adhering material cannot be assessed visually.

The dull putty knife is referenced in surface cleanliness standards besides hand and power tool cleaning as well, including:

SSPC-SP 7, Brush-Off Blast Cleaning (and SP 7 [WAB]),

SSPC-SP14/NACE No. 8, Industrial Blast Cleaning (and SP 14 [WAB]),

SSPC-SP16, Brush-off Blast Cleaning of Coated and Uncoated Galvanized Steels, Stainless Steels, and Non-Ferrous Metals,

SSPC-SP18, Thorough Spot and Sweep Blast Cleaning for Industrial Coating Maintenance.

Again, the use of any visual guides (e.g., SSPC-VIS 1 for dry abrasive blast cleaning) should be supplemented with a dull putty knife to confirm the adherence of remaining material.

Uncovering a Conundrum: While preparing this blog I uncovered the fact that only SSPC-SP 2 and SSPC-SP 3 have the dull putty knife definition included in the body of the standard. All the remaining standards that reference a dull putty knife don’t define it; they just say to use one. And the definition in the latest published version of the SSPC Glossary (2011) defines the dull putty knife differently than what is in SSPC-SP 2, SP 3 and the unpublished 2018 SSPC Glossary. It is unlikely that an inspector would think to seek out the most current definition of a dull putty knife in SSPC-SP 2 or SP 3 (or even know that it exists for that matter) on a project that invokes SSPC-SP 7 (or SP 7 [WAB]), SP 14 (or SP 14 [WAB]), SP 16, or SP 18. So, it is conceivable that the latest approved definition and characteristics of the dull putty knife may not be adopted. If an upcoming project involves SSPC-SP 7, SP 14, SP 16, or SP 18, it would be worth discussing this issue with all parties in advance, and explain that the dull putty knife, as defined in SSPC-SP2 and SP3, has been updated since the 2011 SSPC Glossary was published.

There are additional challenges to using a putty knife.  One involves the amount of force applied to the blade when testing.  The flexibility of the blade helps to mitigate the use of excessive force somewhat, but force is something that has not yet been quantified. The putty knife also shouldn’t be used to chip at the coating to determine its adherence. Further, while the dull putty knife is used to test the edges of remaining material, it must be recognized that areas away from the edges are not being tested.  As a result, it is possible that the edge of remaining paint, for example, is intact, but the middle is loosely adherent.  Because there’s no edge in the middle to test, this problem could escape detection.  Although not part of any standards, you might consider tapping the surface lightly to see if the coating flexes or you hear a hollow sound.  Random adhesion tests (per ASTM D3359[1] or D6677[2]) can also help to determine the integrity of the coating, but they shouldn’t be performed unless allowed by the specification. Finally, since not all putty knives are created equal despite the definitions, I have seen specifications instructing the contractor to select the dull putty knife for the project, to avoid potential conflicts with different dull putty knives on a single job producing different results.

As AMPP or FROSIO trained and/or certified coatings inspectors, it is our responsibility to know the industry standards as well as the inspection tools used to determine conformance to the specification. One of the lesser recognized inspection tools is the dull putty knife. While it is likely the lowest cost tool in your inspection kit, it may be one of the most important on a maintenance painting project.

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[1] ASTM D3359, Standard Test Methods for Rating Adhesion by Tape Test

[2] ASTM D6677, Standard Test Method for Evaluating Adhesion by Knife

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