When a coating system fails prematurely, it is rarely because of the coating itself. Either the surface preparation was inadequate, the surface was contaminated, or the coating was mixed or applied incorrectly. As the old adage goes, a house is only as strong as the foundation it is built on. When it comes to coatings, surface preparation and surface cleanliness are the foundation, and both can significantly impact the performance of the coating system.
Application of a coating over dust will prevent it from adhering to either the substrate or the previous coat. To prevent the coating from peeling, flaking, or other types of adhesion loss, the surface must remain clean. But just how “clean” does the surface need to be? A common statement in painting specifications or product data sheets (PDS) will include some variation of “a 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 foreign matter.” For this article, we will focus on visible dust.
Visually assessing the substrate for dust can be difficult since there is no gage that quantifies surface dust concentrations. The lack of contrast of dust on steel can further complicate the assessment. Wiping the surface with a bare hand will contaminate the prepared surface with moisture, oils, and salts from perspiration. Even wiping with a gloved hand or white cotton cloth is not recommended as fibers from the cloth can catch on the roughened (blast cleaned) steel, gloves may be contaminated with dirt or oils and the pressure used, and distance wiped, is inconsistent and therefore not meaningful. So how is “visually free of dust determined?”?
The Surface Dust Standard
For any test to be meaningful, it must be performed in a consistent manner, which is why there are consensus-based standards in the coatings industry that may be invoked in contract documents. The standard to assess dust on cleaned steel surfaces is ISO 8502, Preparation of Steel Substrates Before Application of Paints 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). With this standard, a simple pictorial rating scale is used to quantify the amount of dust and the average size of particles in a consistent manner.
First, the standard defines dust as “loose particulate matter present on a steel surface prepared for painting, arising from blast-cleaning or other surface preparation processes, or resulting from the action of the environment.” Because dust is loose particulate and not tightly adhered to the surface, in sufficient quantities, it may interfere with the bond of the coating to the substrate.
The test is composed of a special clear, pressure-sensitive adhesive tape that is pressed onto the steel substrate, removed, and placed on a contrasting background. The contrast allows for the evaluator to visually examine the amount of dust adhered to the tape. The type of tape is an important component as not all tapes adhere the same. So how does the test work?
- The adhesive tape is virtually colorless and transparent. The width is about one inch wide (25mm) and has a very specific adhesion peel strength.
- The display board with contrasting color to the dust. Black or white paper works well for this.
- The calibrated spring-loaded roller that can apply a consistent, specific load (optional).
- A 10x magnifying lens.
Remove a piece of the tape, approximately 8” long (200mm) from the roll and apply approximately 6” (150mm) of the center of the piece of tape on the steel. Apply the tape using the calibrated roller by moving from one end of the tape to the other three times. Be sure to maintain the proper downward load at a constant speed so that each stroke takes between 5 and 6 seconds. Thumb-pressure can be used in lieu of the spring-tensioned roller, provided all parties agree. Remove the tape from the surface and place it on the appropriate colored display board. Be sure that all the tape and adhesive is removed from the surface prior to painting.
Assess the quantity of dust on the tape and compare it to the pictorial reference in the standard, with 1 being the least amount of surface dust to 5 representing the highest quantity of surface dust. Using the aide of 10x magnifier, classify the size of dust particles from 0 being not visible to 5 being larger than 2.5 mm in diameter. The specification must indicate the acceptable level of dust quantity and classification. The specification will also need to indicate the frequency of testing (i.e., number of tests per square foot of prepared steel).
Alternatives to ISO 8502-3
As mentioned earlier, most project specifications or the coating manufacturer’s written instructions will invoke a requirement to remove surface dust prior to coating. However, if ISO 8502-3 is not specified, it cannot be used since there is no corresponding acceptance criteria (quantity and classification of size). Even if a standard is not explicitly referenced in the specification to test the surface for dust concentrations, some reasonable assessment to determine whether the surface is “free of visible dust” will still be required.
First, visible dust must be defined. According to SSPC-SP 1, “visible” means detectable with normal eyesight or corrected vision without the use of additional test equipment, such as magnification. Using the definition from the ISO 8502-3 standard, dust is a loose particulate present on the steel. Therefore, the surface is visually inspected for loose particulate that can be seen without magnification.
Because this is a visual assessment, adequate lighting is paramount as you cannot inspect what you cannot see. SSPC Guide 12, “Guide to Illumination of Industrial Coating Projects” provides information on minimum and recommended levels of lighting. The recommended minimum for inspection is 50-foot candles. A foot candle (fc) is a unit of illumination (1 fc = 1 lumen/ ft2) as measured on a surface that is one foot from, and perpendicular to, a uniform point source of one candle.
Prior to coating application, dust is typically removed by a “blow down” using compressed air. Of course, the cleanliness of the compressed air must be checked (ASTM D4285, Standard Test Method for Indicating Oil or Water in Compressed Air) to confirm that it is not introducing moisture and oil to the surface. Proper ventilation/dust collection must be utilized during a blow down to remove the contaminants from the air, otherwise the surfaces will become re-contaminated.
The surfaces need to be carefully examined at this point as dust may not be removed due to static electricity. A light wipe of a gloved hand or clean, dry paint brush should not leave evident tracks on the surface. If it does, the surface has sufficient quantity of dust remaining to potentially interfere with adhesion and cause premature failure. If sufficient dust remains, the surface will require additional treatment such as a second blow-down, or other methods of dust removal including brushing, vacuuming, or even SSPC-SP 1, Solvent Cleaning.
Will Surface Tolerant Coatings Tolerate Surface Dust?
The performance characteristics of the coating system and the prevailing service environment primarily drive the degree of surface cleanliness. Coating systems have varying degrees of surface tolerance, and some are more forgiving than others; however, it is important to understand what surface tolerance really means. Coatings that are surface tolerant can be applied over tightly adhering materials such as mill scale, corrosion, existing paint. Flaking mill scale, loose rust, and paint, along with abrasives and surface dust are not considered “tightly adhering” and surface tolerant coatings will not perform if applied to these surface conditions. Surface tolerant coatings cannot tolerate surface dust any differently than coatings that are not surface tolerant. The surfaces must still be free of visible dust and dirt immediately prior to coating application
Minimizing Surface Dust
There are many sources of dust on an industrial site. Areas where painting operations are taking place should be contained using tarpaulins or other suitable barriers to prevent dust from affecting the application of paint. If abrasive blast cleaning is specified for surface preparation, there are a few considerations to help minimize the dust created from this operation.
During abrasive blast cleaning there are two primary sources of airborne contamination, including the mill scale/rust/coating being removed from the steel and the abrasive used for blast cleaning. The greater the breakdown rate (friability) of an abrasive, the higher the amount of airborne dust. Expendable (single use) abrasives generally have a high breakdown rate and correspondingly high dust generation properties. Therefore, proper ventilation/dust collection is necessary. The containment and dust collection system should be designed to efficiently remove the airborne dust quickly so that it less likely to settle on the prepared surfaces. SSPC Guide 16, “Guide to Specifying and Selecting Dust Collectors” provides guidance on selecting dust collection systems.
Vacuum shrouded tools can also be utilized to reduce airborne dust. Vacuum abrasive blast cleaning is not as productive as conventional abrasive blast cleaning and is typically used for small localized areas. The blast nozzle is fitted into a shroud assembly that surrounds only the nozzle and equipped with a vacuum. The tools need to be fitted with the correct shrouds (based on surface configurations) and the seal between the blast nozzle and substrate must be maintained to minimize dust emissions, which is not always feasible.
Wet abrasive blasting can also be used to suppress dust emissions. The particulates are wetted and do not disperse in the atmosphere. The use of water aids in the removal of non-visible contaminants (e.g., soluble salts) as well. Wet abrasive blasting has its limitations as well and may not always be feasible. Rust inhibitors that are compatible with the coating system could be used to prevent the development of flash rust on prepared surfaces.
Surface dust that is not removed can cause coatings systems to fail prematurely, but this type of failure is preventable. There are several ways to minimize and remove surface dust prior to coating application. Before applying any coating, the surface must be assessed to verify it is truly ready to be painted. A well-prepared coating specification will define acceptable dust levels. While “dust-free” isn’t practical, a qualitative test for visible surface test as described in this article should be employed prior to application. Properly prepared surfaces will allow the coatings to bond and perform as intended.