Coating application is the construction process of uniformly applying coating to the surface of an object to be coated using specific equipment and process methods, thereby forming an anti-corrosion coating. Common coating methods include brushing, knife coating, spraying, flow coating, air spraying, and high-pressure airless spraying.

1. Brushing

Brushing is the earliest, simplest, and most traditional manual coating method. It is convenient and flexible to operate and can be used to coat objects of any shape. Except for coatings that dry quickly and have poor leveling properties, it is suitable for various coatings. The brushing method enables the coating to penetrate the fine pores on the metal surface, enhancing the adhesion of the coating to the metal.

Generally speaking, brushing can be divided into three steps: application, leveling, and finishing. Common defects of brushed coatings include sagging, brush marks, bubbles, pinholes, and uneven thickness. In addition, brushing is labor-intensive and has low work efficiency.

2. Knife Coating

Knife coating is a coating method that uses a scraper. It is also a commonly used coating method, mainly applied in puttying processes. Scrapers can be made of wood, steel, horn, rubber, and other materials. At present, some high-solids coatings and solvent-free, high-viscosity coatings with high solid content are applied by knife coating, such as glass flake coatings.

Common defects of knife-coated films include cracking, peeling, and curling. It is also difficult to achieve uniform film thickness. Cracking of knife-coated films is generally caused by applying a thickness in a single operation that exceeds the allowable thickness of the coating, or by incompatibility with the substrate.

3. Dip Coating

Dip coating is also a traditional coating method. In this method, the object to be coated is immersed in a tank containing coating, then removed so that excess coating drips back into the tank, or the excess coating is removed mechanically. This method is suitable for coating equipment or workpieces with complex structures. The equipment is simple and the degree of mechanization is relatively high. However, it is not suitable for coatings with high volatility. When such coatings are applied by dip coating, solvent loss is significant and can easily cause air pollution.

The disadvantages of dip coating are that the coating thickness is difficult to make uniform, sagging can easily occur, and bubbling may also be produced.

4. Flow Coating

Flow coating is a coating process in which a pressure or gravity nozzle forms liquid coating into a stream and sprays it onto the surface of the object to be coated. The coating flows from top to bottom and completely covers the surface, forming a coating film.

Flow coating was developed as an improvement over dip coating. Both methods rely on excess coating passing over the surface of the object to form a coating film; the difference lies only in how the coating is applied to the surface. The equipment used for flow coating is simple, making mechanized production relatively easy to achieve. Operation is convenient and production efficiency is high.

Flow coating consumes more solvent than dip coating, but requires a smaller initial quantity of coating. Workpieces suitable for flow coating are generally larger than those suitable for dip coating, such as large panels. However, the workpiece itself must not be too complex in shape and should not have pits, grooves, or other areas where coating can easily accumulate. Like dip coating, flow coating can be operated intermittently or continuously. The most typical flow-coating equipment is through-type equipment suitable for large-volume continuous production. It mainly consists of a booth, coating tank, solvent tank, spray unit, and ventilation and fire-protection system. The spray unit consists of nozzles, spray pipes, and a coating pump. The nozzles are installed in the flow-coating area, 300～400mm from the workpiece to be coated. The nozzles may be circular or fan-shaped, and the spray pipes should be designed according to the shape of the object to be coated.

The flow-coating process may also create safety and pollution problems. To prevent solvent evaporation and diffusion, suction-type air curtains should be installed on both sides of the inlet and outlet of the spray booth. The suction air velocity of the air curtain should be greater than the solvent evaporation velocity. An automatic fire-extinguishing system should be provided to prevent fire.

Common coating defects in flow coating include an uneven or incomplete coating, uneven coating thickness, and coatings that are too thick or too thin.

5. Spraying

Spraying can be divided into two methods: air spraying and airless spraying. Its advantages are uniform coating thickness, a smooth appearance, and high production efficiency. Its disadvantages are that material loss is much greater than with brushing, flow coating, and other methods, and the use of solvent-based coatings can cause environmental pollution. Spraying is suitable for various coatings and objects to be coated and is the most widely used coating process.

5.1 Principle of Air Spraying:

Air spraying uses the negative pressure generated when compressed air is discharged from the spray gun nozzle to draw coating from the coating container. The coating then immediately enters the high-speed compressed airflow, causing the liquid and gas phases to rapidly diffuse. The coating is dispersed into mist particles and projected toward the object to be coated. The mist particles are evenly deposited on the surface, forming a continuous coating film.

Brief Summary of Air-Spraying Process Issues

1. High air pressure produces fine paint-mist particles and greater mist dispersion, resulting in significant coating loss;

2. Low air pressure produces coarse paint-mist particles, a rough coating surface, orange peel, pinholes, and other defects;

3. The amount of coating discharged is limited by the air volume. Increasing the discharge volume makes the paint-mist particles coarser and affects the smoothness of the coating surface;

4. The width of the spray pattern should be adjusted according to the condition of the object to be coated. A narrow pattern reduces spraying efficiency, while an excessively wide pattern increases mist dispersion and coating loss;

5. During spraying, use an overlap of 1/3 or 1/2

5.2 Characteristics of Air Spraying

The characteristics of air spraying are as follows:

(1) High coating efficiency: the spraying area is approximately 150-200㎡ per hour, 10 times higher than brushing;

(2) Good coating quality: the coating is uniform and its adhesion to the coated surface is improved;

(3) Strong applicability: it is suitable for almost all types of coatings and objects to be coated;

(4) High coating loss and low coating utilization (50% or less), resulting in environmental pollution.

5.3 Main Equipment for Air Spraying and Types of Spray Guns

The main process equipment for air spraying consists of a spray gun, air compressor, oil-water separator, air receiver, and coating supply equipment. According to the coating supply method, it can be divided into gravity-feed, suction-feed, and pressure-feed types.

5.4 Introduction to Airless Spraying Machines

5.4.1 Principle of High-Pressure Airless Spraying Machines

High-pressure airless spraying continuously delivers coating into a closed pipeline through a pressure pump, thereby forming high pressure in the enclosed space. When the trigger of the spray gun connected to the end of the coating pipe is released, the coating collides with the air at a speed of 100m/s at the moment it is discharged, rapidly expanding and atomizing.

5.4.2 Characteristics of the Airless Spraying Process

(1) A thick coating film can be applied in a single pass. Various high-solids and high-build coatings can be sprayed to obtain a relatively thick coating film;

(2) High spraying efficiency, approximately 3 times that of air spraying, reaching 400-1000㎡/h;

(3) High coating utilization (with no air atomization, coating dispersion is reduced), thereby reducing environmental pollution;

(4) It can be used to spray various two-component heavy-duty anti-corrosion coatings, including epoxy, polyester, polyurethane, and polyurea coatings.