Discuss how to carry out the surface preparation work for painting

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Surface pretreatment before painting is as important for the coating as the foundation of a building; the quality of the foundation determines whether the building is safe or not. No matter how good the paint is, applying it to an untreated surface is pointless. It can sometimes accelerate the damage to the underlying surface. Surface pretreatment can create conditions for the coating in two aspects: one is mechanical, by providing surface roughness for the coating ； The second aspect is chemical; it ensures close contact between the molecules of the coating and the surface of the steel substrate. 1. The adverse effects of smooth surfaces on coatings: When the surface to be coated is very smooth, there is no good adhesion between the coating and the surface. For example, we can easily remove the coating from a glass surface using a scraper or even our fingernails. On the contrary, the painted surface is as rough as sandpaper, making it difficult to remove the coating. After being treated with shot blasting (sand), the surface of steel becomes as rough as sandpaper, with numerous tiny peaks and valleys, which indicate the surface roughness. The coating extends deep into the bottom of the wave troughs, with the wave crests acting like teeth that firmly grip the coating. In the coating industry, some people describe surface roughness as “anchors” or “mechanical teeth”. 2. Harmful effects of surface contaminants on coatings
Contaminants on painted surfaces include: dirt, dust, grease, oil, rust, scale, and sometimes moisture. If these contaminants are covered by a coating, the mechanical and chemical adhesion of the coating will be affected, causing the coating to fail. On the contrary, removing dirt before painting ensures better adhesion of the coating to the painted surface. A coating with good adhesion acts as an effective barrier, preventing moisture from coming into contact with the steel. 3. Harmful substances invisible to the naked eye: There are also some chemical hazards that cannot be seen with the naked eye. The most harmful are certain soluble salts, such as chlorides and sulfides, etc. If these harmful substances are not completely removed, they will allow water vapor to penetrate the coating and reach its bottom, causing bubbling and separating the coating from the substrate, which in turn accelerates the corrosion of the steel. During the surface preparation of corroded steel structures prior to painting, the areas where corrosion pits appear after shot blasting (especially the bottoms of these pits) may contain soluble salts. Dry shot blasting cannot remove these salts completely; therefore, it is advisable to use specialized on-site testing equipment before painting to check whether soluble salts are present on the surface of the workpiece and what their concentration is. If the concentration of soluble salts exceeds the allowable level, measures must be taken to remove them. Only by fulfilling the above points can a solid foundation be laid for anti-corrosion coating of a project.