Traditionally, coating equipment generally only has coating materials with protective and decorative properties, and these properties are usually one-way. Therefore, the surface treatment of metal before painting must be done through degreasing, rust removal, and phosphating (oxidation) to ensure the basic performance of the product.

The quality of surface treatment before painting (also known as pretreatment process) is an important link that directly affects the service life and decorative effect of the coating. The purpose of pretreatment before painting is to remove foreign matter such as oil, rust, etc. on the surface of the substrate, provide a clean surface suitable for painting, and significantly improve the adhesion and corrosion resistance of the coating.

Degreasing methods are divided into physical and mechanical methods and physical and chemical methods. The former includes wiping method, burning method and spraying method. The degreasing methods used before automobile painting are mainly physical and chemical methods, including alkali degreasing method, emulsion degreasing method and solvent degreasing method. Alkali degreasing method mainly removes grease through saponification. When removing mineral oil that cannot be saponified, the action of surfactants is used to help emulsification and degreasing. This method is widely used, has good degreasing effect, and is more economical. The emulsion degreasing method is to add one or more surfactants to an organic solvent, or to add a weak three-property detergent to form a mixed liquid. When the emulsion is impregnated or sprayed on the object to be cleaned, the solvent penetrates the grease to micronize the grease, and the surfactant emulsifies the grease microparticles and disperses them in the cleaning liquid, thereby removing the grease. There are usually two types of solvent degreasing methods: immersion method and trichloroethylene cleaning method. The immersion method is a method of immersing the object to be cleaned in an organic solvent and degreasing it by stirring or manually washing. Commonly used solvents are gasoline, xylene, acetone, etc. The trichloroethylene cleaning method is also called dry cleaning method. Usually, trichloroethylene is heated and trichloroethylene vapor is used to degrease the object to be cleaned. Rust removal is to remove the oxides and hydroxides generated on the metal surface. The method of rust removal is usually selected according to the color, generation state and procedure of the rust. Rust removal methods can be divided into physical rust removal and chemical rust removal. Physical rust removal includes manual rust removal and mechanical rust removal. Chemical rust removal often uses pickling rust removal. Manual rust removal usually uses tools such as scrapers, scrapers, rust hammers, wire brushes and emery cloth, so the labor intensity is high, the work efficiency is low, the quality is unstable, and the labor protection is poor, which is not suitable for application in automobile manufacturing. However, it is still the simplest and commonly used method in the automobile maintenance industry. Commonly used tools for mechanical rust removal include hand-lift electric grinding wheels, electric brush wheels, pneumatic brushes, and pneumatic rust removal wheels with needle beams. At present, mechanical rust removal mostly uses sandblasting and shot blasting. Chemical rust removal is commonly used in pickling, that is, immersing the metal in an acid solution so that the oxides and hydroxides attached to the metal surface undergo a chemical reaction and dissolve in the acid solution, thereby removing the rust.

Phosphating is a method in which the metal surface is subjected to a certain chemical treatment to form an insoluble, phosphated complex salt protective film on the metal surface. Phosphating is widely used in the automotive industry for covering parts, cabs, body panels, etc. As a surface treatment before painting, it can increase the adhesion and corrosion resistance of the coating. Commonly used phosphate coatings are divided into two categories: zinc phosphate type and manganese phosphate type. Zinc phosphate type films are thin, dense, smooth, have a certain degree of corrosion resistance, and can withstand certain deformation, but the bath liquid is not stable enough and maintenance is more complicated. Manganese phosphate type, on the other hand, has the characteristics of stable bath liquid and easy maintenance.

Precautions for spraying metallic paint

(1) After spraying metallic paint, dust must be removed before the next operation.

(2) Metallic paint mainly plays a covering role. Do not spray too thickly, otherwise the paint color will be too dark.

(3) Stir evenly. Metal particles such as aluminum powder in metallic paint are easy to settle at the bottom of the paint can, so metallic paint needs to be stirred more thoroughly before spraying than general natural paint.

(4) The matching diluent and the corresponding dilution ratio must be selected according to the instructions provided by the paint manufacturer, and the viscometer should be used correctly to master the required viscosity to ensure that the paint color meets the requirements.

(5) Use of spray gun. When spraying metallic paint, uniform paint film thickness is very important. The nozzle should be aimed at the spraying surface at a vertical angle, and the distance between the two should be kept between 5 and 20 cm. Move the spray gun in a straight line at a uniform speed, and avoid moving the spray gun in an arc. Each reciprocating spraying requires a 50% overlap area, so that a fixed color can be obtained. When the amount of paint output of the spray gun needs to be adjusted, the spray gun trigger can be pressed to the bottom, and the speed of the spray gun movement can be used to increase or decrease the thickness and dryness of the paint film.

The main purpose of phosphating treatment for primer before painting is to improve the adhesion of the paint film and the corrosion resistance of the coating system, so the focus is on the matching performance with the paint film. Generally, the quality inspection indicators of phosphating include film appearance, film thickness and performance after matching with the paint film. The appearance of the film should be a uniform, fine and complete phosphating film. For light iron phosphating, its appearance should be a uniform, fine and complete phosphating film. For light iron phosphating, its appearance should be a complete red and blue color film.