Surface Preparation: Key to Protective Coating Excellence
In the realm of protective coatings, the adage “preparation is key” couldn’t ring truer. Whether safeguarding a water storage tank against the elements or enhancing the lifespan of wastewater treatment process equipment, applying protective coatings is only as effective as the surface preparation performed before it is applied. Proper surface preparation lays the foundation for a durable and long-lasting coating system, dictating its adhesion, performance, and resilience over time. From preventing contamination to mitigating corrosion, the efficacy of protective coatings in the water and wastewater sector hinges upon meticulous surface preparation practices. In this article, we discuss the significance of surface preparation and provide an overview of industry standards.
Improperly prepared surfaces can reduce coating integrity and service life. According to Sherwin Williams, a leading coating manufacturer, up to 80% of all coating failures can be directly attributed to inadequate surface preparation, which affects coating adhesion. Surface preparation is equally important for the performance of protective coatings on metal and concrete surfaces in the water and wastewater industries. The three key steps to surface preparation involve: 1) pre-cleaning to remove surface contaminants, 2) cleaning the surface to the desired level, and 3) producing a surface profile (texture) for good adhesion of the coating. It is important to note that structural repairs and/or modifications should be completed before surface cleaning occurs. This includes grinding welds or other sharp edges and cuts, filling pits in steel, and patching or filling surface voids in concrete.
Surface Contaminants
The presence of surface contaminants threatens coating performance and longevity. They can hinder proper adhesion, accelerate coating degradation, and ultimately lead to premature failure. Understanding surface contaminant types, sources, and effects is paramount in the water and wastewater industry, where protective coatings often provide the first line of defense against corrosion. Surface contaminants that may cause early deterioration of coatings are described in the list below.
Rust - This is the corrosion product of steel (iron oxides). Rust, whether loose or relatively tightly adherent, does not provide a good base for applying coatings because it expands and becomes porous when the iron reacts with water and oxygen. Therefore, it must be removed for reliable coating performance.
Mill Scale - Mill scale is a bluish, shiny oxide residue that forms on steel surfaces during hot rolling. Unless it is completely removed before painting, the mill scale will cause the coating to crack and expose the underlying steel. Steel is anodic to mill scale and will corrode more rapidly due to this combination of ‘dissimilar metals.’
Grease and Oil - A thin, not readily visible film of grease and oil will prevent a coating from adhering to the substrate tightly.
Dirt and Dust - Dirt and dust also prevent tight bonding of coatings and must be removed completely before coating application.
Water - Steel and concrete surfaces must be dry before painting. Water can prevent an organic coating from properly “wetting out” the surface on metal or concrete surfaces. Moisture may produce flash rusting on a metallic surface before coating or accelerate underfilm corrosion after coating application. Concrete structures that were previously immersed will never dry completely, and moisture should be considered when selecting coating systems.
Soluble Salts - Soluble salts are deposited from the atmosphere onto exterior surfaces, especially in coastal areas. They can interfere with proper bonding between the substrate and the coating. Soluble salts absorb moisture from the environment or the coating itself, leading to osmotic blistering. Soluble salts can also accelerate underfilm corrosion.
Paint Chalk - A powdery substance that forms on paint's surface over time, particularly in exterior applications. The chalky substance results from paint degradation due to exposure to sunlight (UV radiation), moisture, or other environmental factors. Loose chalk can hinder proper adhesion and must be removed before coating.
Deteriorated Paint - All old, loose paint must be removed before topcoating or recoating a surface. Before removing old paint, it should be determined whether it contains lead or other toxic materials. If toxic materials are present, special precautions may be needed to protect workers and the environment.
Surface Preparation Methods
The most commonly accepted standards for contractors, coating experts, and agencies in the water and wastewater industry are the NACE International/Society for Protective Coatings (SSPC) Joint Surface Preparation Standards. NACE and SSPC have recently merged to form the Association for Materials Protection and Performance (AMPP). The SSPC standards and coating product data sheets provide guidelines for achieving specific surface cleanliness and profile requirements, thereby facilitating the proper adhesion and performance of protective coatings. Some of the most common SSPC surface preparation standards include:
SSPC-SP-1: Solvent Cleaning: This standard outlines the requirements for solvent-cleaning surfaces to remove oil, grease, dirt, and other loosely adherent soluble contaminants. It specifies the types of solvents to be used, such as steam, emulsifying agents, or other cleaning compounds, as well as the application methods.
SSPC-SP-2: Hand Tool Cleaning: This standard details the procedures for cleaning steel surfaces using non-powered hand tools, such as wire brushes, scrapers, or sandpaper. These tools are typically employed to remove loose rust, mill scale, paint, and other contaminants that may negatively impact coating application.
SSPC-SP-3: Power Tool Cleaning: This standard describes using power tools, such as power sanders or grinders, to clean surfaces to a specified level of cleanliness. SSPC-SP-3 is suitable for removing rust, mill scale, and coatings from steel surfaces where hand tool cleaning is impractical. Power tools are frequently used for spot cleaning damaged coatings where contamination of adjacent areas by abrasive cleaning is unacceptable.
SSPC-SP-5/NACE No. 1: White Metal Blast Cleaning: This SSPC/NACE joint standard describes cleaning a previously painted or unpainted steel surface to a white metal condition using abrasive blast media. This is one of the most rigorous surface preparation standards and is used to achieve a clean, bare metal surface devoid of all (0%) visible residues, contaminants, and coatings. This level of cleanliness is often required for high-performance coating systems in immersed environments.
SSPC-SP-6/NACE No. 3: Commercial Blast Cleaning: This joint standard also describes cleaning a steel surface using abrasive blast media. Like a white metal blast cleaning, the surfaces shall be free from all visible oil, grease, dust, dirt, mill scale, rust, coating, and other foreign matter. Randon staining from previous exposure to contaminants is acceptable so long as the contamination does not exceed 33% of any 3-inch by 3-inch surface area. This standard is commonly used for non-immersed coating applications.
SSPC-SP-10/NACE No. 2: Near-White Metal Blast Cleaning: This standard conveys the requirements for cleaning a steel surface, painted or unpainted, using abrasive blast media. It includes instructions for achieving and verifying the standard’s required end condition. As with commercial blast cleaning, the prepared surface must be free of visible contaminants except for a limited amount of acceptable staining. Unlike commercial blasts, only 5% of any 3-inch by 3-inch surface area may exhibit staining or minor discolorations from previous exposure. This standard is commonly used for immersed steel surfaces.
SSPC-SP-11: Power Tool Cleaning to Bare Metal: This standard contains the requirements for power tool cleaning steel to produce a bare metal having a minimum 1 mil (0.001 inch) deep surface profile. This is very useful for field preparation of small steel areas when abrasive blasting is not feasible due to dust concerns or cost/schedule.
SSPC-13: Surface Preparation of Concrete: This standard contains the requirements for concrete preparation by mechanical, chemical, or thermal methods.
It is up to the specifier to select the appropriate surface preparation method based on the service environment and project constraints. The list of SSPC standards provided is not all-inclusive, and it is imperative to know how to use the standards appropriately. The entire library of SSPC surface preparation standards is an essential reference for coating specifiers, contractors, and inspectors to ensure consistent and effective surface preparation practices are followed.
The significance of proper surface preparation in coating applications cannot be overstated. By adhering to recognized standards such as those set forth by AMPP, the water and wastewater industries can achieve consistent and reliable results, safeguarding critical infrastructure and assets against the harmful effects of corrosion. Investing time and resources into thorough surface preparation enhances the performance of protective coatings and yields long-term cost savings by minimizing premature recoating and repairs. Ultimately, prioritizing proper surface preparation underscores a commitment to quality, durability, and sustainability in coating applications.
At V&A, infrastructure preservation is at the core of our mission. Protective coatings offer the first line of defense in extending and preserving infrastructure service life. Our coatings experts assist water and wastewater clients nationwide in selecting the proper coating or lining system to mitigate damage caused by corrosive environments. We also assist clients in preparing coating specifications and outlining effective surface preparation methods required for long-term results.
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References
AMPP. (2021, September 20). Surface Prep Standards - A Quick Summary. Retrieved from AMPP: https://blogs.ampp.org/surface-prep-standards-a-quick-summary
Sherwin Williams. (n.d.). Surface Preparation. Retrieved March 2024, from Sherwin Williams: https://www.sherwin-williams.com/architects-specifiers-designers/products/resources/surface-preparation