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Whether in automobile manufacturing or repair and refurbishment, the final effect of metal surface coating depends not only on the preliminary "foundation work"—degreasing, rust removal, and phosphating—but also on the "precise operation" of the subsequent spraying. If oil or rust remains on the metal surface, even the best subsequent paint application will result in problems such as paint peeling, blistering, and fading; and small oversights during spraying can also lead to uneven paint color and poor texture. This article will break down the core steps of the entire automotive painting process in detail: from choosing different methods for degreasing and rust removal, to the technological value of phosphating, and to 5 key considerations for metallic paint spraying. It uses easy-to-understand language to explain the principles, characteristics, and practical techniques of each step, helping you avoid painting pitfalls and create a durable and beautiful paint finish.
Oil stains (such as mineral oil and grease) on the metal surface will seriously affect the adhesion of the coating and must be thoroughly removed. Degreasing methods are mainly divided into physical-mechanical methods and physical-chemical methods. The latter is most commonly used before automotive painting, as it is more targeted and has more stable results.
These methods remove oil stains through physical action. They are simple to operate and suitable for small-area, light oil stain treatment:
* Wiping method: Use a cloth soaked in solvent to directly wipe the oil stains on the metal surface. Suitable for local cleaning.
* Combustion method: Remove oil stains through high-temperature combustion, but this can easily damage the metal surface and is only suitable for special scenarios.
* Spraying method: Use high-pressure airflow or high-pressure water to spray the surface, using impact force to peel off the oil stains. Suitable for large areas of light oil stains.
These methods utilize chemical action to decompose or emulsify oil stains, resulting in more thorough degreasing and meeting the high requirements of automotive painting. They are mainly divided into three types:
1.2.1 Alkaline Degreasing Method
The core principle is to decompose grease through "saponification." For saponifiable oils such as animal and vegetable oils, the alkali solution can directly decompose them into water-soluble substances; while for non-saponifiable mineral oils, the surfactants added to the alkali solution will play an "emulsifying role," breaking down the oil stains into tiny particles that are dispersed in the solution and carried away. This method has particularly prominent advantages: good degreasing effect, wide applicability, and low cost. It is the most widely used degreasing method before automotive painting, effectively cleaning both body panels and parts.
1.2.2 Emulsion Degreasing Method
Simply put, this involves adding one or more surfactants to an organic solvent, and sometimes combining them with a weakly alkaline detergent to create a mixed cleaning solution. When using this method, the emulsion is immersed or sprayed onto the metal surface. The organic solvent quickly penetrates the oil, micronizing it, while the surfactant emulsifies and disperses these tiny oil droplets in the cleaning solution, thus thoroughly removing the oil. This method combines the rapid degreasing ability of organic solvents with the emulsifying and dispersing effect of surfactants, making it suitable for handling workpieces with complex shapes or stubborn oil stains.
1.2.3 Solvent Degreasing Method
The solvent degreasing method removes oil stains by the dissolving action of organic solvents. There are two main operating methods: Immersion method: The workpiece to be cleaned is directly immersed in organic solvents such as gasoline, xylene, and acetone. Degreasing is achieved by stirring or manual rinsing. This method is simple to operate and suitable for batch processing of small parts. Trichloroethylene cleaning method (also known as dry cleaning): Trichloroethylene is heated to generate steam, which is then used to degrease the workpiece. The advantages of this method are fast degreasing speed, less residue, and the solvent can be recycled and reused, making it suitable for precision parts with high cleanliness requirements.
Oxides and hydroxides (rust) on metal surfaces can damage the bond between the coating and the substrate, and must be thoroughly removed. Rust removal methods can be divided into physical rust removal and chemical rust removal based on their principles. The choice depends primarily on the color, formation state, and severity of the rust.
Physical rust removal removes rust through mechanical force or manual operation without altering the properties of the metal substrate. It mainly includes two types:
2.1.1 Manual Rust Removal
This involves manually scraping or grinding rust using tools such as putty knives, scrapers, rust hammers, wire brushes, and sandpaper. The advantage of this method is its simplicity and lack of specialized equipment, making it the most convenient and commonly used method in the automotive repair industry. However, its disadvantages are also obvious: high labor intensity, low work efficiency, inconsistent rust removal quality, and insufficient protection for operators, making it unsuitable for large-scale mass production in automobile manufacturing.
2.1.2 Mechanical Rust Removal
Mechanical rust removal utilizes the mechanical force generated by specialized equipment to efficiently remove rust. Common tools include handheld electric grinders, electric brush wheels, pneumatic brushes, and pneumatic rust removal wheels with needle bundles. Currently, the most widely used methods in automobile manufacturing are sandblasting and shot blasting:
Sandblasting: High-speed abrasive particles impact the metal surface, quickly removing rust and creating surface roughness, which is beneficial for subsequent coating adhesion.
Shot blasting: A high-speed rotating impeller propels abrasive particles onto the metal surface, resulting in strong impact and high rust removal efficiency. It is suitable for processing large areas and batches of workpieces and can achieve automated production.
The most common chemical rust removal method is pickling: The metal workpiece is immersed in an acidic solution, where a chemical reaction dissolves the oxides and hydroxides on the surface, thus thoroughly removing the rust. The advantages of this method are fast rust removal, uniform effect, suitability for handling complex-shaped workpieces, and simultaneous removal of surface oxide scale. However, it is crucial to control the pickling time and acid concentration to avoid excessive corrosion of the metal substrate. After pickling, neutralization and cleaning are necessary to prevent residual acid from affecting coating quality.
Phosphating treatment is a key "pretreatment process" before automotive painting. Through specific chemical treatment, an insoluble phosphating complex salt protective film is formed on the metal surface. This thin film plays a vital role: it significantly enhances the adhesion of subsequent coatings, prevents coating peeling, and improves the corrosion resistance of the metal surface, extending the service life after painting. In the automotive industry, phosphating treatment is widely used in key components such as body panels, cabs, and body panels, and is one of the core processes ensuring the quality of body painting. Currently, commonly used phosphating films are mainly divided into two categories, each with its own characteristics:
The film layer is thin, dense in structure, and smooth in surface, possessing certain corrosion resistance and the ability to withstand certain deformations (such as bending after car body stamping). It is suitable for workpieces requiring subsequent forming processes; however, its bath stability is relatively poor, and daily maintenance is relatively complex, requiring regular monitoring and parameter adjustment.
The biggest advantage is the stability of the bath solution, convenient daily maintenance, high phosphating efficiency, and excellent wear resistance and adhesion of the film layer. It is suitable for large-scale mass production in automobile manufacturing and is currently the preferred solution for many car companies.
Metallic paint spraying is the "final key process" in automotive painting, and the operational details directly affect the color, gloss, and durability of the paint surface. Here are 5 important points to remember:
After spraying metallic paint, the surface must be thoroughly dusted before the next application (such as clear varnish). During the drying process, metallic paint easily absorbs dust and particles from the air. Applying subsequent coats without dust removal will result in a grainy finish, affecting the appearance.
The core function of metallic paint is to "cover the base color and present a metallic texture." The thickness must be controlled during spraying; it should not be too thick. If the coating is too thick, the paint color will become too dark and dull, losing the proper luster and texture of metallic paint. It will also prolong the drying time and easily lead to defects such as runs and orange peel.
Metallic paint contains metallic particles such as aluminum powder. These particles easily settle at the bottom of the paint can, resulting in an uneven metallic texture on the paint surface. Therefore, metallic paint must be thoroughly stirred before spraying to ensure that metallic particles are evenly dispersed in the paint liquid. If necessary, a stirring tool can be used to assist, avoiding insufficient manual stirring.
Different brands of metallic paint have different compatible thinners and dilution ratios. It is essential to strictly follow the paint manufacturer's instructions for selecting the thinner and adhering to the recommended dilution ratio. At the same time, it is crucial to correctly use a viscometer to measure the paint viscosity to ensure it meets the spraying requirements—too high a viscosity will result in uneven spraying, while too low a viscosity will cause sagging; both will affect the consistency of the paint color.
The operation of the spray gun is the core of metallic paint spraying, directly determining whether the paint film thickness is uniform and the color is consistent: The nozzle should be aimed at the spraying surface at a vertical angle, with the spraying distance controlled between 5 and 20 cm. Too close, and it can easily lead to excessively thick local coatings; too far, and it will affect the adhesion of the paint mist. When moving the spray gun, maintain a uniform linear movement, avoiding an arc-shaped movement, otherwise it will result in uneven paint thickness. Each back-and-forth spray should maintain a 50% overlap area (for example, if the width of the first spray is 10 cm, the second spray should cover the previous 5 cm) to ensure uniform paint color and no missed spots. When adjusting the paint output, the spray gun trigger can be pulled all the way down. By controlling the speed of the spray gun movement, the paint film thickness and drying rate can be adjusted—the faster the movement, the thinner the paint film and the faster the drying; the slower the movement, the thicker the paint film and the slower the drying.
Automotive painting is a complex, interconnected system. From basic cleaning (degreasing and rust removal) to protective priming with phosphating, and finally to fine finishing with metallic paint, every step is crucial: For degreasing, choose the right method. Depending on the type of oil and the workpiece, prioritize alkaline degreasing, emulsion degreasing, or solvent degreasing to ensure thorough cleaning. For rust removal, balance efficiency and quality. Automotive manufacturing prioritizes mechanical rust removal (sandblasting, shot blasting), while automotive repair can flexibly use manual rust removal for emergencies. Phosphating is essential; it's key to improving coating adhesion and corrosion resistance. Choose zinc phosphate or manganese phosphate phosphate films based on workpiece requirements. For metallic paint spraying, meticulous attention to detail is crucial. Even mixing, thickness control, and standardized spray gun operation are essential to create a uniform, aesthetically pleasing, and durable finish. Whether for automotive manufacturers or repair shops, mastering these core processes and practical points can effectively prevent common problems such as paint peeling, blistering, and uneven coloring, improving painting quality and efficiency. For consumers, understanding these processes helps them better judge the quality of painting techniques and choose more reliable products or services. In short, there are no "shortcuts" in automotive painting; only by meticulously handling every detail can a metal surface be both "good-looking" and "durable."
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