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General instructions for surface treatment: When using coatings to protect steel, it is crucial to carry out thorough surface treatment. If this step is neglected, optimal performance cannot be achieved. Surface treatment consists of primary surface treatment (aimed at removing oxide scale, rust, and foreign substances from the steel surface before coating) and secondary surface treatment (aimed at removing rust from the steel surface before applying anti-rust paint, as well as removing any foreign substances from the surface even if a shop primer has already been applied). The steel surface can be rust-removed using the following methods. Sandblasting: Any rusty or unclean surfaces can be effectively cleaned by scrubbing. Granular abrasives (such as sandblasting media and sand particles) are propelled at high speed by a medium, usually air, and sprayed onto the surface to be treated in order to remove rust and dirt. In terms of production efficiency, abrasives with a size of 0.3–1.5 mm are the most effective, especially when it comes to achieving the required standards when treating pitted surfaces. The treatment speed using slag spraying is usually faster than that using sand particles. Local sandblasting: Hulls that have rust on small areas sometimes require local sandblasting in order to effectively remove the rust and bring the surface cleanliness up to standard. In practice, certain measures must also be taken to prevent future damage: A/The paint film around the sandblasted area may be cut by the abrasive, and the paint film on the edges may loosen from the surface; if this happens, it is necessary to use a grinding disc to completely remove the loose paint layer from those edges. B/ Coatings in the surrounding area may be splashed with occasional abrasive particles, which could damage the protective properties of the nearby coatings. To obtain a coating with good protective properties, the damaged areas around the area to be sandblasted must be cleaned; surfaces between various areas that require local sandblasting can also be damaged if sandblasting is not done carefully. Therefore, when sandblasting is moved from one area to another, the spraying must be stopped; in the event of damage under such circumstances, it should be repaired in the aforementioned manner. C/ It is recommended to mark the sandblasting areas with chalk, if possible. Sandblasting: When using mineral sand to treat large areas of a ship’s hull or compartments, different surface conditions arise. Surfaces that are coated with paint or have mild rust can be easily treated to reach the Sa2.5 standard. However, surfaces with severe rust or pitting are more difficult to treat, and it may not be possible to achieve the Sa2.5 standard in such cases. Through numerous controlled experiments and by studying different particle sizes of materials, it was found that the most effective abrasive size for treating rusted steel surfaces is mineral slag with particle sizes ranging from 0.3 mm to 1.5 mm. After sandblasting, the dust on the surface must be removed. In an open environment, the exterior of the hull can be cleaned using dry compressed air blown from a spray gun; however, inside the hull care must be taken, and a vacuum cleaner should generally be used to thoroughly remove all sand particles and dust. Sand blasting: Sand blasting is a treatment method that involves the rapid spraying of abrasives; its effectiveness depends on the properties and condition of the surface, as well as the type of abrasive and the size of its particles, in addition to the skill of the operator. A) Light sweeping: Rapid sweeping sandblasting can remove dirt or loose coatings from the surface; it can be used to abrade existing hard paint coats in order to improve the adhesion of new coatings. This method is effective for mild corrosion on steel surfaces that have been coated with a shop primer and are subject to wind erosion, but it cannot achieve a cleanliness level of Sa2.5. The size of the abrasive particles is important – to avoid damaging the paint surface, particles ranging from 0.2 mm to 0.5 mm in size are most suitable. B) Used to remove the aging coating from the surface of steel plates so that the primer layer or the bare steel surface is exposed. When the standards for the exposed steel surface vary, all standards are acceptable as long as the rust is removed. The high-intensity blasting method can also be used for surfaces coated with traditional anti-fouling paints, on ships that need to switch to copolymer anti-fouling paints. The particle size of the abrasive grains is the same as that described in the lightly swept area. High-pressure fresh water flushing: The main procedure involves spraying high-pressure fresh water directly onto the surface. Similar to shot blasting, the cleaning efficiency depends on the natural condition of the surface, as well as the water pressure and the distance between the nozzle and the surface. The injection angle also affects the effectiveness; for surface dirt or seaweed, a water pressure of 140–210 kilograms per square centimeter is generally used, as high-pressure water may be ineffective against shellfish. Even if coatings with poor adhesion are removed, the hard coating will not be damaged even under a water pressure of up to 350 kilograms per square centimeter. Hydraulic jetting: Hydraulic jetting involves adding abrasives to the water flow; this method can be used to effectively remove remaining tough paint layers and rust, yielding satisfactory results. There are various devices available on the market, and they differ in the way in which the abrasives are mixed with the water. The water pressure is usually set at 210 kilograms per square centimeter; care should be taken in selecting the abrasive and adjusting the water pressure. This method is limited to removing the failed anti-fouling paint without damaging the surface of the rust-proof paint; if the underlying paint is damaged, it is repaired. After hydraulic jetting, water rust usually forms on the steel plates; moist surfaces are highly prone to oxidation. Operators can use certain rust inhibitors to prevent this water rust, but our company does not approve of such methods at present. It is necessary to remove all this water rust with fresh water before applying any coatings. It is recommended to use an electric brush to clean off the rust from the surface, and the remaining ginger-colored surface can be covered with most exterior coatings. Electric roller brush: Its brush is made of steel wire, and it uses the twisting force that occurs upon surface contact to polish away rust rather than removing it from the surface. Since rust is difficult to remove, this method cannot clean the surface thoroughly. However, this method still has some value in dealing with mild rust resulting from hydraulic jetting and early-stage rust on steel plate surfaces coated with a shop primer. Electric roller: This method is quite effective for treating surfaces in most types of painting applications; a silicon carbide pad is used to remove rust from the surface of steel plates. To penetrate into the pitted areas, a conical silicon carbide tip is necessary. This method can be effective for completely removing rust in localized areas, but it requires considerable manual effort when dealing with overhead surfaces. Although this method can theoretically be used on larger areas, the endurance of the workers limits its applicability; therefore, the operator’s skills affect the quality of the treatment, and strict control is thus essential. Mechanical chisels: These tools are usually pneumatic; they use vibrating needles or chisels to strike the surface, thereby removing rust. This method is more effective than manual removal, but rust is only removed in the areas where the chisel has made contact, with some rust still remaining on the surface. Where chiseling has taken place, a shiny steel surface appears, but areas that have not been chiselled still retain a layer of rust. Manual treatment: The term “scraper” refers to the use of sharp blades, along with wire brushes and chisels, for rust removal. This is a traditional method of rust removal, but its effectiveness is not ideal. For years, scrapers have been used to remove loose rust and paint debris, but they cannot eliminate firmly attached rust or the salts contained within it. Chisels can only remove small areas of rust; a significant portion remains on the surface. The remaining rust layer is often the most chemically active part. Manual wire brushes can remove loose, powdery forms of light rust, but they cannot remove oxide scales. Surface treatment standards: When precise specifications are required for the cleanliness and degree of rust on the surface of steel before painting, our company adopts the Swedish standard SIS055900 for sealing off oxidized scale or rust-covered steel sheets. This standard describes the rust grade as follows: A: The steel surface is completely covered by scale, with very little rust present if any. B: Rust begins to form on the steel surface, and as a result, the oxide scale starts to flake off. C: The oxide scale on the steel surface has rusted off, but very slight pitting can be seen with the naked eye. D: The oxide scale on the steel surface has rusted off, and a considerable amount of pitting is visible to the naked eye. Swedish standards define six levels of surface treatment, three of which are adopted in accordance with our company’s technical requirements: St3 – thorough removal of oxide layers, rust removal using wire brushes, mechanical rust removal, and polishing. These treatments eliminate loose oxide layers, rust, and foreign particles; after cleaning with a vacuum cleaner, clean compressed air, or a cleaning brush, the surface should exhibit a distinct metallic roughness and luster. Sa2.5: A very thorough sandblasting cleaning process, which should remove oxide scale, rust, and foreign substances to such an extent that only minimal imperfections in the form of spots or streaks remain. The surface is then cleaned using a vacuum cleaner, clean compressed air, or a cleaning brush. Sa3: Sandblasting purification leaves the surface as pure metal