Main content of surface treatment before car painting

     The degreasing method is divided into two types: physical mechanical methods and physical chemical methods. The former has a wiping method, a burning method, and a spraying method. The degreasing method used for automobile coating is mainly a physicochemical method, including an alkali degreasing method, an emulsion degreasing method, and a solvent degreasing method. The lye degreasing method mainly removes fats and oils by saponification. When removing mineral oil that cannot be saponified, it relies on the action of surfactant to help emulsification and degreasing. This method is widely used, and the degreasing effect is better and more economical. The emulsion degreasing method is a mixture of one or several surfactants added to an organic solvent, or a weak trisodium detergent. When the emulsion is impregnated or sprayed on the object to be cleaned, the solvent impregnates the grease to atomize the grease, and the surfactant emulsifies and disperses the grease particles in the cleaning liquid to remove the grease. There are two types of solvent degreasing methods: impregnation and trichloroethylene cleaning. The dipping method is a method in which the object to be cleaned is immersed in an organic solvent and degreased by stirring or manual washing. Commonly used solvents are gasoline, xylene, acetone, and the like. The trichloroethylene cleaning method is also called a dry cleaning method. Usually, trichloroethylene is heated, and the object to be washed is degreased with trichloro vapor.

    Derusting is to remove oxides and hydroxides from metal surfaces. The method of rust removal is usually selected according to the color of the rust, the state of formation, and the procedure. The rust removal method can be divided into physical rust removal and chemical rust removal. Physical derusting has two types: manual descaling and mechanical descaling. Chemical descaling is commonly used for pickling and rust removal. Manual descaling usually uses tools such as blade, scraper, rust hammer, wire brush and abrasive cloth, so it has high labor intensity, low work efficiency, unstable quality and poor labor protection, and is not suitable for use in automobile manufacturing. But it is still the easiest way to use it in the auto repair industry. Commonly used tools for mechanical rust removal include hand-exposed electric grinding wheel, electric brush wheel, pneumatic brush and pneumatic derusting wheel with needle bundle. At present, the mechanical rust removal method adopts sandblasting rust removal method and shot blasting method. Chemical descaling is commonly used in acid washing, in which a metal is immersed in an acid solution, and the oxide and hydroxide attached to the metal surface are chemically reacted and dissolved in the acid solution to remove rust.

    Phosphating treatment is a method in which a metal surface is subjected to a certain chemical treatment to cause a metal surface to be infiltrated with an insoluble, phosphated double salt protective film. Phosphating is widely used in the automotive industry for coverings, cabs, body panels, and the like. As a surface treatment before coating, the adhesion and corrosion resistance of the coating film can be increased. Commonly used phosphating membranes are zinc phosphate type and manganese phosphate type. The zinc phosphate type film is thin, dense and smooth, has certain corrosion resistance, and can withstand certain deformation, but the bath liquid is not stable enough and the maintenance is complicated. The manganese phosphate type has the characteristics of stable bath and convenient maintenance.

    Metal paint spraying precautions

    (1) After spraying the metal paint, it must be dusted before the next operation.

    (2) The metal paint mainly serves as a covering effect. Do not spray too thick, otherwise the color of the paint surface will be too dark and too dark.

    (3) Stirring should be uniform. Metal particles such as aluminum powder in the metal paint are easily deposited on the bottom of the paint can, so the metal paint needs to be thoroughly stirred before the general paint.

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

    (5) Use of the spray gun. A uniform film thickness is very important when spraying metallic paint. The nozzle should be aligned with the spray surface at a vertical angle. The distance between the two should be kept between 5 and 20 cm. The spray gun should be moved linearly at a constant speed to avoid moving the spray gun in an arc. Each reciprocating spray requires 50% overlap to achieve a fixed color. When it is necessary to adjust the amount of paint sprayed, the gun can be pulled to the end and the speed of the spray gun can be used to increase or decrease the thickness and dryness of the paint film.