(1) brush coating
Brushing is the earliest, simplest and most traditional manual painting method. It is easy to operate and flexible. It can be applied to all kinds of paints except those with fast drying and poor leveling. The brush coating can permeate the pores of the metal surface and enhance the adhesion of the coating to the metal. Disadvantages are labor intensity, low efficiency and poor coating appearance.
Scraping is a method of using a scraper to coat liquid coatings with high viscosity and 100% solid content. The common defects of the coating in scraping operation are cracking, shedding, rolling, etc. The thickness of the coating is also difficult to be uniform.
(three) dip coating
Dip coating is also a traditional coating method. The method immerses the coating in the bath containing the paint, and then takes it out to let the surplus paint drip back into the bath, or uses mechanical method to throw the surplus paint off. The solvent loss of this method is large and easy to cause air pollution. It is not suitable for volatile coatings, and the thickness of coatings is not uniform. It is generally used for equipment or workpieces with complex structure.
For example, in chemical plants, some equipments, such as assembled heat exchangers, can be constructed by integral dipping because the inner tubes can not be brushed and sprayed. Aluminum-dipped steel pipes are mainly used for tubular furnace pipes, heat exchanger pipes and fractionator pipes in petrochemical industry.
Spraying is a process engineering in which paint is sprayed from nozzle to the surface of the coating, and the coating is completely covered by the surface of the coating through top-down flow. The equipment used in this method is simple, easy to realize mechanized production, easy to operate and high production efficiency, but the film is not uniform or covered incompletely, and the film thickness is not uniform. Larger consumption of solvent than dip coating will cause safety and pollution problems.
Spraying is to use the negative pressure produced by compressed air in the nozzle of the gun to eject the paint from the nozzle of the paint container, and quickly enter the high-speed compressed air flow, so that the paint liquid one by one vapor phase diffuses rapidly, and is dispersed into foggy particles to shoot at the coated object, evenly coated on the surface. This method requires that the viscosity of the sprayed coating is relatively small. Diluent is often added to the coating. The advantage of the spraying method is that the film thickness is uniform, the appearance is smooth, and the production efficiency is high. The disadvantage is that the material loss is much greater than that of brushing and spraying methods, and the use of solvent coatings will cause environmental pollution. It is suitable for all kinds of coatings and various coating materials. It is the most widely used painting process. For example, the spray coating used in chemical production is commonly used to spray aluminum and zinc on carbon steel. Zinc coating is mainly used to prevent atmospheric and water corrosion. Aluminum plating can be used in the corrosive environment of sulfur compounds, such as sulfur-containing petroleum processing equipment and rubber vulcanization tanks.