"OTB-TASC" provides a true visual picture of surface cleanliness.
The Farlex Free Dictionary defines clean as “free from dirt, stain, foreign matter, impurities”. In the industrial surface preparation/coating application arena, a clean surface is of the utmost importance for coating adhesion and life expectancy. Manufacturers of high performance coating materials recognize this requirement and make a statement on their product data sheets similar to “ surface must be clean and free of ALL dirt, dust, laitance(concrete), oil, grease and any other contaminants”. Obviously, they are cognizant that correct surface preparation, anchor profile, proper coating selection and a clean surface, when combined in a project can produce desired results.
Let’s discuss these components to a successful coating project highlighting surface cleanliness.
Correct surface preparation: Steel or concrete are the two most common materials coated in the industrial field. Many variables dictate the method employed prior to coating application. Whatever method used, the optimum purpose of surface preparation is to produce an adequate anchor profile for the greatest possible coating adhesion.
Anchor Profile: Even though there are many types and grades of steel, surface profile is usually obtained by abrasive blasting. To provide a roughened concrete surface for coatings, various methods are employed such as grinding, acid etching, abrasive blasting, shot blasting and scabbling. The degree of suitable roughness or surface texture is dependent upon the coating/coating system to be installed. The most obvious thing about concrete is…it’s not steel! It is completely different and yet is many times thought of and treated the same. Concrete, because of its chemical makeup, is more prone to coating failure. Unlike steel, concrete is very porous with air, water, water vapor and contaminants going in, coming out and sometimes staying within. One of the most important characteristics for a coating or an overlayment’s bonding is the texture or “profile” of the substrate. With concrete, the upper portion of a slab surface is often called the anchor profile or surface profile. It is a measure of the surface roughness. Adhesion of a coating to concrete is primarily by two methods: Mechanical or Absorption. It can be by both as well. The mechanical method involves the adhesion of the coating to the aggregate. The chemical absorption method involves the absorption of the polymer into the cement binder. Along with this absorption is a certain amount of mechanical adhesion to the aggregate. Most polymers do not penetrate the aggregate.
Adhesion: Adhesion is defined in Webster’s New World College Dictionary 4th Edition as “the force that holds together the molecules of unlike substances whose surfaces are in contact”. When it comes to coatings and adhesion to a substrate, that “holding together” is extremely important. The types of substrates are endless, but in the high performance coating industry it usually narrows down to two, metals or concrete. For either one, the correct coating must be applied over the properly prepared surface. “Surface Preparation and Coating of Concrete” defines surface profile, sometimes referred to as anchor profile, as “The roughened surface that results from abrasive blast cleaning or power tool cleaning. In concrete, surface profile is the texture of the cleaned surface”.Within Webster’s dictionary, the word “textured” is defined as “uneven, not smooth”. “Peak to valley“ is another way of explaining surface profile. Peak density is not covered in this article. Prior to coating application both visual and mechanical methods should be employed to assess the surface and its profile. Surface cleanliness as well as surface profile will greatly affect coating adhesion.
Coating selection: The specification should state the coating selected for the project at hand. This selection may be based on prior use on other same type projects or coatings suggested by a coating manufacturer. Either way, the product data sheets, as a general rule, will set forth guidelines for surface preparation and cleanliness. Surface cleanliness and the means of inspection of/testing for, is an issue that behooves our perusal.
Clean surface: ISO Tape test Standard: ISO 8502-3 “Preparation of steel substrates before application of paint and related products-Tests for the assessment of surface cleanliness. Part 3: Assessment of dust on steel surfaces prepared for painting (pressure-sensitive tape method).
Under Scope, section 1.1, the paragraph reads “This part of ISO 8502 describes a method for the assessment of dust remaining on cleaned steel surfaces prepared for painting. It provides pictorial ratings for the assessment of the average quantity of dust. It also provides descriptive classes for the assessment of the average size of the dust particles.”
Section 1.2 reads, “The method may be carried out either
- as a pass/fail test by assessing the quantity of dust present on a test surface, and the average dust particle size, in comparison with specific limits; or
- to provide a permanent record of the dust present on the surface by mounting the tapes used to carry out the tests on tiles, cards or paper, of an appropriate contrasting colour”.
Section 1.3 reads, “This method is suitable for the assessment of dust retained, after cleaning, on a steel surface which corresponded before cleaning to rust grade A, B or C as defined in ISO 8501-1. Because of the limited elasticity of adhesive tape, it is not possible for the tape to penetrate into the deep pits present in cleaned steel that originally corresponded to rust grade D.”
Section 6.1 “At the beginning of each series of tests, remove and discard the first three turns of tape from the roll and then remove a piece about 200mm long.”
Section 6.2 reads in part,”Touching the adhesive side of the tape only at the ends, press 150 mm of the freshly exposed tape firmly on to the surface under test.”
Section 6.3 reads, “Assess the quantity of dust on the tape by comparing visually an area of the tape with equivalent sized areas of the pictorial references shown in figure 1. Record the rating corresponding to the reference that is the closest match.”
Section 6.4 reads, “Assess the predominating dust particle size on the adhesive tape by reference to table 1 which defines 6 dust particle size classes, designated 0,1,2,3,4 and 5.”
Section 6.5 reads, “Carry out a sufficient number of tests to characterize the surface under test. For every surface of one particular type and aspect, carry out not less than three separate tests. If the results do not have a spread of one or less quantity rating, carry out at least two additional tests to establish the mean.”
Although this test method is discussed and taught in coating inspection classes (NACE, SSPC) it has been this author’s experience that in the USA, this test method or any other test method for surface cleanliness is rarely inserted into coating specifications. It could be possible that the boilerplate specifications, used so frequently by specifying engineers, never contained any method to test for surface cleanliness or the assumption is that the contractor/coating applicator will clean the surface as directed by the coating manufacturers data sheets. Even certified coating inspectors, unless otherwise directed, may skip this test as well. It would certainly behoove these individuals writing the surface preparation/coating application specifications to require a cleanliness test to be taken and recorded. Inspectors should, if possible at a pre-job meeting, ask about any tests that should be taken to ensure that the surface is properly cleaned and then recorded for future reference.
As of now, this author is not aware of any test kit on the market that is available for other surfaces, such as non-ferrous materials, plastic, sheetrock or concrete. The clear tape test has been available for approximately 30 years and is still missing from so many specifications. Possibly, the test is thought of as only a lab test or not sufficiently assessing the surface cleanliness. It is this author’s opinion that although the test has merits, there is an alternative approach that although similar, is easier to use, easier to decipher and record the results for a myriad of substrates. That method is “TASC”.
“TASC”: “Tape Assessment of Surface Contaminants”
In contrast to the abovementioned clear tape test, “TASC” employs:
- The use of a two-sided adhesive foam tape, white in color, usable on any desired substrate such as steel, concrete, plastics, sheetrock, etc.
- A roller to press tape into the surface profile being tested.
- A visual percentage chart employing six (6) visual graduated percentage amounts that allow both immediate visual comparison and recording/retention of results. (Percentage results have been verified by the Utah State University
- Understandable pass/fail results for documentation by contractor/inspector/owner/QA/QC
- Savable/recordable results for present and future interdiction
In contrast to a clear tape test that sits on the top of the anchor profile, the foam tape utilized by “TASC” removes contaminants from within the profile, thus providing a true visual picture of the surface cleanliness. This method, if stipulated by coating manufacturer, specifying engineer or owner within the specification will definitely aid in achieving a specified surface cleanliness, aid in coating adhesion, system longevity and customer satisfaction. With “TASC” the statement “you’ll know when the “task” is done” can become a reality.