How to apply coating using the compressed air spraying method? What are the characteristics of this coating method?

Air spraying is a coating method in which compressed air, after oil and water separation, is fed into a spray gun to create a negative pressure that forces the paint in the paint tank to be ejected and atomized, thereby forming a coating on the surface of the product. 　　Spraying methods include manual spraying, manual automated line spraying, two-component spraying, thermal spraying, and gas fusion spraying. Air spraying is suitable for coating products of various shapes, sizes, and materials. It offers high efficiency and good operability, enabling the creation of a uniform and aesthetically pleasing paint finish. Therefore, although various automatic painting methods are continuously being developed and applied in the automotive industry. 　　However, the air spraying method is applicable to almost all types of coatings and substrates, and therefore remains widely used in automobile painting. The disadvantage of this method is that it cannot be used with coatings having too high viscosity; it consumes a large amount of solvent, and significant amounts of solvent are dispersed during the atomization process. The utilization rate of the coating is only 40% to 50%, resulting in severe environmental pollution. It is a coating method that urgently requires technical improvement. Currently, a low-pressure air spray gun has improved the utilization rate of paint. 　　The air spraying method involves a spraying apparatus that includes a spray gun, a compressed air supply and purification system, as well as a paint delivery system; the painting station also needs to be equipped with a dust removal and air conditioning system, as well as systems for exhaust ventilation and removing paint mist. Based on the basic requirements for automotive coatings, the primer is typically applied using the electrophoretic method, or a primer and topcoat are applied together; the topcoat is then applied using air spraying or electrostatic spraying.

Compressed air at a pressure of 3-5 Pa is used to atomize the paint, which is then sprayed onto the surface of the object to be coated to form a paint film. During spraying, the nozzle should be perpendicular to the surface to be coated, and it should move horizontally up and down without any swaying. When using the spraying method, it is essential to ensure that the surface to be coated remains very clean and dry; otherwise, the adhesion of the paint will be reduced. There is significant dispersion of paint caused by compressed air, as well as a lot of solvent evaporation, so special attention must be paid to sparks. The spraying method is efficient and suitable for painting large areas, but paint splatter leads to some loss of paint. Due to equipment limitations, this method is generally used only in shipyards.

Compressed air spraying is a coating method that uses a special paint gun to atomize the paint into a fine mist through the flow of compressed air, which is then sprayed onto the surface of an object. A uniform paint film can be formed on the surface of the object that is sprayed. In general, the paint film obtained through spraying is smoother and more aesthetically pleasing than one applied by hand brushing. The main advantages and disadvantages of spray coating are as follows: (1) When using spray coating, even on surfaces with pores, slopes, curves, or sand marks and irregularities left after grinding, a uniform coating can be applied, resulting in a smooth and even paint finish. (2) High construction efficiency, 5-10 times faster than the brushing method. Especially for large-area objects, spraying is more effective. (3) Some paints have low flowability but dry quickly, such as nitrocellulose paints (commonly known as \"spray paint\") and vinyl chloride paints. When applied using a brush, these paints often leave brush marks on the coating, which affects its appearance, and they also result in low application efficiency. If spraying is used for construction, such phenomena can be prevented. (4) The disadvantages of spray painting are: 1. The paint consumption is as high as 40% ； 2. The coating thickness per application should not be too thick; therefore, the number of application passes increases ； 3. A large amount of solvents are released during construction, causing severe environmental pollution and posing a risk of fire and explosion. Therefore, safety and labor protection measures are more stringent. Improvement measures and technical approaches for spray coating include: (1) Using thermal spraying, which involves heating the paint to 60–70°C in order to reduce its viscosity. This reduces the amount of diluent used, thereby increasing the content of film-forming substances in the paint and also reducing the number of application coats required. (2) Spraying is carried out in a specially designed painting booth, and the airborne paint mist can be captured using recovery devices; this allows for the recovery of some of the material while also preventing the risk of fire and explosion during spraying. (3) In the past, solvents and thinners used for painting were mainly benzene-based, but they are now gradually being replaced by benzene-free solvents. Benzene-free solvents or diluents have a slightly higher cost, but they pose less harm to the human body.

Air spraying, also known as gas spraying or conventional spraying, is a method that relies on the flow of compressed air to create negative pressure at the nozzle, thereby drawing the paint out of the paint tank and turning it into a mist. With the help of this air flow, the paint is applied to the surface of the object to be coated. Air spray equipment is simple, easy to operate, and convenient to maintain. It features high coating efficiency and good workability, producing a uniform and attractive paint film. It can coat 150–200 m2 per hour (about 8–10 times that of brushing), making it suitable for spraying ordinary coatings. The disadvantage of air spraying is that the paint must be prepared according to the specified ratio before spraying, so as to achieve an appropriate viscosity; during spraying, a considerable amount of paint is lost as it disperses with the air ； The film formed is relatively thin, and multiple spraying operations are required to achieve the specified film thickness. The penetration and adhesion of coatings are generally worse than those achieved by brushing ； Paints and solvents that spread in the air are harmful to human health and the environment ； In poorly ventilated areas, when the vapor of solvents reaches a certain level, it can cause explosions and fires. (1) Equipment and tools for air spraying method. The air spraying method apparatus includes the following. 1 Spray gun, which creates a mist of paint and sprays it onto the surface of the object to be coated. 2 Compressed air supply and purification system (air compressor): Supplies clean, dry, oil-free compressed air. 3 Paint feeding device, which stores the paint and supplies it continuously. 4 rubber hoses, for conveying compressed air. 5 Paint spraying rooms should be equipped with ventilation and safety devices; the ideal indoor temperature is 18–30°C, with a relative humidity of less than 70%. (2) Methods and techniques of air spraying. The quality of painting work depends mainly on the viscosity of the paint, the working pressure, the distance between the nozzle and the surface, as well as the operator’s skill level. To achieve a smooth, even, and uniform coating, it is necessary to master the correct spraying techniques.