What are the best practices for surface preparation before applying coatings and finishes?

1 Types of surface preparation

There are three main types of surface preparation: solvent cleaning, mechanical cleaning, and chemical cleaning. Solvent cleaning involves using a solvent, such as acetone, to remove oil, grease, dirt, and other contaminants from the surface. Mechanical cleaning involves using abrasive tools, such as sandpaper, wire brushes, or blasting media, to remove rust, mill scale, and other loose materials from the surface. Chemical cleaning involves using acids, alkalis, or salts to dissolve or convert the surface contaminants into soluble or inert compounds.

2 Methods of surface preparation

The method of surface preparation depends on the type and condition of the surface, the type and thickness of the coating, and the desired level of cleanliness and roughness. The most common methods of surface preparation are hand tool cleaning, power tool cleaning, abrasive blasting, and water jetting. Hand tool cleaning uses manual tools, such as scrapers, chisels, or hammers, to remove loosely adhering materials from the surface. Power tool cleaning uses electric or pneumatic tools, such as grinders, sanders, or needle guns, to remove firmly adhering materials from the surface. Abrasive blasting uses compressed air or water to propel abrasive particles, such as sand, steel grit, or glass beads, against the surface to create a uniform and rough profile. Water jetting uses high-pressure water to remove dirt, grease, and loose materials from the surface.

3 Standards of surface preparation

The standards of surface preparation specify the degree of cleanliness and roughness required for different types of coatings and environments. The most widely used standards are the ISO 8501-1 for visual assessment of steel surfaces, the SSPC-SP for surface preparation specifications by the Society for Protective Coatings, and the NACE for surface preparation standards by the National Association of Corrosion Engineers. These standards classify the surface preparation into different grades or levels, based on the amount and type of visible contaminants and defects on the surface.

4 Cleaning the welds

The welds are the most critical areas for surface preparation, as they are prone to defects, such as porosity, cracks, slag inclusions, and spatter, that can compromise the coating performance and integrity. The welds should be cleaned thoroughly to remove any weld flux, slag, spatter, oxide layers, or other contaminants that can interfere with the coating adhesion or cause corrosion. The welds should also be smoothed and blended with the surrounding metal to avoid sharp edges or irregularities that can create stress concentrations or coating defects.

5 Degreasing the surface

The surface should be degreased before and after mechanical or chemical cleaning to remove any oil, grease, or other organic residues that can contaminate the surface or the coating. Degreasing can be done by using a solvent, such as acetone or xylene, or a detergent, such as trisodium phosphate or sodium hydroxide, and wiping or rinsing the surface with clean water or air. Degreasing should be done in a well-ventilated area and with proper personal protective equipment to avoid health and safety hazards.

6 Profiling the surface

The surface should be profiled to create a rough and uniform texture that can enhance the coating adhesion and coverage. The surface profile depends on the size, shape, and hardness of the abrasive particles used for mechanical cleaning or blasting, and the pressure and angle of the impact. The surface profile should match the type and thickness of the coating, as well as the service conditions and environment. The surface profile can be measured by using a comparator, a replica tape, or a profilometer.

7 Here’s what else to consider

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