Body surface coating refers to the coating film coated on the outermost layer of the workpiece, which is the only visible part of the coating assembly and plays the role of decoration, marking and protection of the substrate. It is in direct contact with various climatic conditions (such as rain, sunshine, snow, cold and hot summer, etc.). ) and harmful substances (such as acid, alkali, salt, carbon dioxide, hydrogen sulfide), and it is the first layer to prevent these corrosion, and it plays a protective role together with other coatings. Of course, different automobile painting quality will have different emphasis on the decorative and protective properties of top coat due to factors such as grade requirements and use environment. For example, cars have high requirements for decoration: trucks loaded with oil, acid and alkali chemicals have high requirements for oil, acid and alkali resistance of topcoat, and decoration is second. Therefore, in order to meet various needs, the coating industry also produces various performance topcoats, which can be mainly divided into the following categories. \x0d\\x0d\ 1, used for high-grade decorative coatings (high-grade car body) \x0d\\x0d belongs to high-grade decorative coatings, which requires excellent decoration, weather resistance and water resistance, and is suitable for various climatic conditions. The appearance of the coating should be smooth, no particles, bright as a mirror, and the luster should be not less than 90%. \x0d\\x0d\2。 X0d used for intermediate decorative coatings (intermediate car) is a high-quality decorative protective coating, which requires excellent decoration, weather resistance and water resistance, and the decoration requirement is only lower than that of high-grade decorative coatings. The mechanical strength is superior to that of high-grade decorative coatings. Suitable for all kinds of climatic conditions. \x0d\\x0d\3。 Used for general decorative coating (automobile body) \ x0d \ x0d \ with good gloss. Hardness, impact strength, flexibility, weather resistance and adhesion are all good, and the gloss and color retention are also good, but the water resistance is poor. 100℃ baking can improve its water resistance and adhesion. \x0d\\x0d\4。 Used for some decorative and protective coatings (automobiles used in hot and humid areas) \ x0d \ x0d \ This kind of coating is bright. It dries quickly and can be polished. Good weather resistance, moisture resistance and aging resistance. It also has certain resistance to damp and heat, salt spray and mildew, and excellent chemical resistance, but its adhesion is poor, so it can not be used for a long time at a high temperature above 60%. \ x0d \ x0d \ 5。 Coatings with characteristic requirements \ x0d \ x0d \ such as acid-resistant coatings, neoprene coatings, chlorinated rubber coatings, epoxy coatings, etc. ; Heat resistant coating. Silicone paint, acrylic paint (thermosetting), etc. Wear-resistant coating, neoprene paint, phenolic paint, etc. Solvent resistant coatings, polyurethane coatings, epoxy coatings, etc. Insulating coatings, asphalt coating, alkyd coating, epoxy coating, etc. \ x0d \ x0d \ The choice of automobile coatings should give consideration to both protection and decoration; We should not only pay attention to the coating of steel substrate, but also consider the coating of aluminum, zinc, plastic and wood substrate. Due to the different surface materials and adsorption capacity of various coated objects, the coatings suitable for them will be different; Therefore, according to the performance and actual requirements of various coatings, we should comprehensively consider various factors and choose the appropriate coatings. \ x0d \ x0d \ The voice of protecting the earth's environment and reducing air pollution has been paid more and more attention, and environmental problems have also strongly affected the development of automobiles and related industries. Therefore, high solidity, water-borne and pulverization have been the basic principles and guiding ideology of developing new products in the world coating industry since the end of 1960s, which has become a reality in western developed countries and continues to develop and improve. \ x0d \ x0d \ waterborne coatings are prepared by using different waterborne resins, using water as solvent and adding only a small amount of alcohol and ether as additives. Waterborne coatings are prone to defects when painting, and their use is limited. The surface tension of water is large and the color dispersion is poor, so it is difficult to obtain high decorative and corrosion-resistant coating properties; Water has a large specific heat capacity and is not volatile, which requires that the temperature and humidity of the painting environment be strictly controlled within the specified range. Generally, it cannot be used in a drying room with solvent-based coatings. \ x0d \ x0d \ The basic principle for selecting topcoat is that \ x0d \ x0d \ (1) is decorative and protective, and meets the product requirements. \x0d\\x0d\(2) Top coat and primer have good compatibility. \ x0d \ x0d \ (3) Suitable for construction conditions. \x0d\\x0d\(4) It is beneficial to reduce the cost and facilitate the construction. \x0d\\x0d\(5) Abide by labor protection laws and regulations to ensure the health of workers. \x0d\\x0d\ I. Classification of surface coatings \x0d\\x0d\ The construction of automobile body surface coatings is generally operated by special personnel. There are many classification methods of surface coatings, which can be divided into solid color paint, metallic paint and pearlescent paint according to color effect. According to the types of film-forming substances, it can be divided into nitrocellulose paint, alkyd paint and acrylic paint. According to the curing mechanism, it can be divided into solvent volatilization type, oxidation type and crosslinking reaction type. \ x0d \ x0d \ In addition, the construction of surface coating can be divided into single process, double process and three process according to the construction technology. However, the boundaries between each classification method are not absolute and can cross each other. \x0d\\x0d\ Single-process surface coating refers to the coating that forms a complete surface coating by spraying the same coating. \x0d\\x0d\ Double-process surface coating refers to a coating that can be sprayed with two different coatings to form a complete surface coating. Usually, the paint is sprayed first, and then the varnish is sprayed. The combination of the two paints can form a complete surface coating with guaranteed quality. \x0d\\x0d\ three processes are complicated. For example, in the three processes, pearlescent paint is generally sprayed with a layer of primer first, then with a layer of pearlescent paint, and finally with varnish, and the three coatings are combined to form a complete topcoat. Generally, the color of single coat is monotonous, and the effect of three topcoats is rich. However, the more processes, the more complicated the construction and maintenance. \ x0d \ x0d \ 1。 Construction of single-process topcoat \ x0d \ x0d \ Single-process topcoat commonly used in automobile maintenance is mainly two-component solid color paint. \x0d\\x0d\ Two-component solid color spray painting is the most popular one in the automobile maintenance industry at present. \ x0d \ x0d \ Preparation of coating surface before spraying: \ x0d \ (1) Requirements for automobile body surface \ x0d \ x0d \ The basic work of primer has been completed on the body surface, and the surface is smooth and free from defects, with clear and accurate contour lines. Use P400-P500. Special degreaser should be used to degrease and wax the coated substrate, and the entire coating surface should be gently wiped with a dust-sticky cloth to remove floating dust. \x0d\\x0d\(2) Inspection and preparation of spraying environment, tools and equipment \x0d\\x0d\ Two-component paint dries slowly and is easy to dust, so the construction environment is required to be clean, and the spraying work should be carried out in the painting room with exhaust equipment. The curing agent isocyanate is very sensitive to water and oil, so the air compressor is required to provide pure and dry compressed air. The spray flow from the spray gun should be uniform. Pay attention to the selection of suitable caliber spray gun. Excessive caliber will lead to a large amount of paint, resulting in sagging and orange peel of the paint film after painting. Use a special filter to filter the paint before spraying to ensure that there are no visible particles on the sprayed paint film. \ x0d \ x0d \ Two-component coatings can reach the required thickness by spraying 2 layers. If the color hiding power is poor, you need to spray 3 ~ 4 layers until it is completely covered. \ x0d \ x0d \ During spraying construction, it should be sealed with intermediate primer to prevent the mismatch between the old and new paint films and provide good durability. And pay attention to determine the properties of the old paint film, to ensure that the old paint film and new paint spraying will not appear problems such as undercut, light absorption, wrinkling and so on. If the surface is partially painted with primer, a thin layer should be sprayed on the primed surface before the first layer of paint is sprayed, so as to reduce the color difference between the intermediate primer and the old surface and lay a foundation for the subsequent spraying operation. \ x0d \ x0d \ When spraying the first layer, spray a thin layer with spraying pressure of 0.35 ~ 0.45 MPa and moderate humidity. Check the coating condition and confirm whether there is shrinkage cavity. Generally, if there is oil or wax on the coating surface, it will cause shrinkage cavity immediately after the first coating. If the primer is not compatible with the newly sprayed paint, it will immediately cause undercut and cracking. At this time, the construction should be stopped immediately, and corresponding remedial measures or rework should be taken. Shrinkage is a common phenomenon in the construction of two-component coatings. This is mainly because the isocyanate in the coating is very sensitive to oil, wax and water. If there is a small amount of shrinkage cavity in the secondary part, after the solvent evaporates, part of the coating is slightly dry, and then the shrinkage cavity is sprayed for 2 ~ 3 times, and the shrinkage cavity disappears when the second layer is sprayed. If the main coating surface or shrinkage cavity is widely distributed, the construction should be stopped, and after drying, it should be polished or scraped with putty according to the specific situation before re-spraying. When slight shrinkage occurs, a small amount of anti-fish eye agent can be added to the primer or topcoat to solve it, but the fundamental solution is to do a good job of surface pretreatment of the substrate. \x0d\\x0d\ After spraying the first layer, if there is no bad situation, it should be left standing for a period of time before spraying the second layer, and the standing time will be a little longer, depending on the ambient temperature and coating type. Generally, when the surface to be coated is not stained with dust, it is the best time to spray the second layer of paint. Master the scale, you can gently touch the paint film in the closed part with your fingers, and spray the second layer if the wet paint film doesn't touch your hands. Generally, after spraying the second layer, the spraying work is over. This layer of spraying requires the coating surface to be smooth and bright, without flow marks and orange peel. Therefore, when preparing the coating, the viscosity is slightly lower than that of the first coating, the moving speed of the spray gun is also slightly slower, and the spraying pressure can be increased by 0.02 ~ 0.03 MPa, so that the coating particles can be atomized more finely to obtain a satisfactory smooth coating. \ x0d \ x0d \ construction notes: \ x0d \ x0d \ 1. Two-component coating is used now, and the prepared coating should be used up within the service time provided by the supplier. \x0d\\x0d\2。 When opening the front door and spraying the back door, the inside of the front door should be sealed with special shielding paper and adhesive tape to prevent paint dust generated during spraying from flying into the car and causing pollution. \x0d\\x0d\3。 When spraying the roof and front cover from the edge to the center, be careful that the work clothes do not touch the sprayed parts of the edge to avoid unnecessary rework. \x0d\\x0d\4。 Carefully remove the protective paper immediately after spraying, and be careful not to touch the wet coating with your hands and work clothes. If it is forced drying, it should be removed in time when the coating is still slightly wet; If the paint film is completely dry, the tape is difficult to remove. On the one hand, it is easy to leave adhesion points, on the other hand, it may be uncovered by adhesive tape. \ x0d \ x0d \ 5。 The isocyanate paint dust in two-component coatings is harmful to human body, so it should be sprayed in a well-ventilated environment, and personal safety protection should be done, such as wearing work clothes and gas masks. \ x0d \ x0d \ 6。 After spraying, the spraying tools should be cleaned immediately to avoid sticking. \x0d\\x0d\7。 Two-component coatings can be self-dried after spraying, and can also be baked at low temperature for forced drying. Generally, it can be put into use after 20℃ (overnight) 16h, baked at 60℃ (metal temperature) for 30min, and then put into use after cooling, but it takes a week to completely cure. Specific products are strictly in accordance with the requirements of suppliers. \x0d\\x0d\ II。 Spraying of double-process topcoat \ x0d \ x0d \ Spraying of double-process metallic paint has the following points. \ x0d \ x0d \ 1 refers to two-component solid color paint, which is used for the preparation of painting surface and the inspection and requirements of spraying environment, tools and equipment. \x0d\\x0d\2 Before spraying, the metallic paint should be evenly mixed. Add curing agent and diluent according to the proportion specified in the coating, generally adjust the viscosity of the coating to15 ~17s (apply -4 cups at 20℃), filter and spray. Use slow-drying curing agent and thinner carefully. \ x0d \ x0d \ 3 Generally, metallic paint is sprayed with 2 ~ 3 layers, which shall be subject to uniform coverage. Each layer should be sprayed evenly, with an interval of 10 ~ 15 min, spraying pressure of 0.4 ~ 0.5 MPa and moderate humidity. Metal particles are easy to sink to the bottom of the spray gun tank, so pay attention to mixing evenly every time you feed, and shake the spray gun frequently when spraying to prevent silver powder particles from settling. \x0d\\x0d\4 The last layer of spraying can appropriately reduce the viscosity of the coating, slightly increase the spraying pressure, and the spraying is thin and uniform, which is beneficial to the uniform distribution of silver powder particles and improve the gloss of the coating. \ x0d \ x0d \ 5 The protective paper and adhesive tape cannot be removed immediately after spraying, but should be removed after drying (usually at 60℃ for 30 minutes, see the product manual of the coating supplier for specific drying conditions) and before completely cooling. \x0d\\x0d\ The above data are for reference only. Please refer to the supplier's requirements for specific products. \ x0d \ x0d \ Three-step pearlescent paint spraying \ x0d \ x0d \ Three-step pearlescent paint spraying needs to spray three different types, namely pure background paint, pure pearlescent paint and varnish, which is called three-step pearlescent paint spraying. For the paint mixing ratio of each step, please refer to the information of the paint supplier. \ x0d \ x0d \ 1 Before spraying, blow dust and stick it, clean the spraying area with degreasing agent, and provide safety measures. \x0d\\x0d\2 Spray three layers of pure background paint to cover the primer, the distance between spray guns is 20cm, the air pressure is 0.3-0.6MPa, and the spraying interval of each layer should have time, so the paint dust of each layer should be cleaned up with a sticky dust cloth. When using dust cloth, each layer of pure background paint needs to be dried to avoid dry spraying. \x0d\\x0d\4 Make sure that the primer is dry for about 30 minutes. Then spray 3 ~ 4 layers of pure pearlescent paint, depending on the color of the car body, the distance of the spray gun is 20cm, and the air pressure is 0.3 ~ 0.6 MPa. During the spraying interval of each layer, it is necessary to clean up the paint dust of each layer with a sticky dust cloth. When using sticky dust cloth, each layer of pure background paint needs dry coal to avoid dry spraying. \x0d\\x0d\4 After spraying multi-layer solid color paint and pure pearlescent paint, the film thickness increases. Therefore, the solvent volatilization is slowed down, so the standing time of varnish spraying is prolonged, and the pearlescent paint is guaranteed to be about 30-60min after drying, depending on the weather temperature. The weather is cold and dry for longer. \x0d\\x0d\ II。 Example of body painting \ x0d \ x0d \ Automobile body painting is a complete process. The following example illustrates the process of painting and repairing the car body. \ x0d \ x0d \ Standard coating construction technology: soapy water cleaning, degreasing, stripping and cleaning of old coating, preparation and filling of putty, polishing (repeated several times according to surface conditions), surface cleaning and preparation of surface coating, surface coating, polishing and waxing. \x0d\\x0d\ 1 Polishing and trimming raw edges, pay attention to the polishing of raw edges, as shown in figure 1. Whether the radian of these parts and the slope of the damaged parts of the coating are suitable or not will directly affect the fullness, luster and refraction of the coating after painting, which is also the main operation method of the coating personnel. \x0d\\x0d\2 In the process of grinding burrs, it is also necessary to check whether there are obvious bumps on the concave surface of the car body. As shown in Figure 2, or the defects that cannot be eliminated after painting, if found, please ask the body sheet metal repairman to shape or repair these defects. \x0d\\x0d\3 The surface of the car body with polished burrs after painting must be free from defects. The range of grinding burrs is much larger than the damage range of the door, and the gradient cannot be too obvious by hand, as shown in Figure 3. \x0d\\x0d\4 Clean the car body, thoroughly remove the stains on the damaged parts of the car door with degreaser, and check and evaluate the damage degree and painting type of the painted objects. Brush off oil and wax with organic solvent. The damaged part of the door has simple structure, smooth surface and easy cleaning. \ x0d \ x0d \ 5 can be used as primer after cleaning, and its main function is to provide adhesion and anticorrosion. Sub-floor generally does not have the ability to fill the surface defects of the car body, so it is relatively easy to operate the curved door smoothly. \ x0d \ x0d \ mixed putty. Firstly, the main ingredients in the tank are mixed evenly, and the viscosity of the bottom surface is consistent, which is convenient for scraping and curing. The curing agent should first open the tube cover to squeeze out air, then screw on the tube cover and rub it with the palm of your hand outside the tube to make the curing agent uniform. When preparing, put the main agent on the tray with a scraper, and add the curing agent according to the ratio of main agent 1% ~ 3% (refer to the instructions for coating use). Scrape putty with a hard scraper. Scraping the first layer of putty is mainly to fill the depression of the car door. Don't scrape repeatedly during operation, each layer can be thinner. The damage of this door is not very serious, as shown in Figure 4. \ x0d \ x0d \ 6 Scrape the second layer of putty, but the body door is flat, so it is scraped with a hard scraper. This layer of putty is mainly filled. The thickness of this layer of putty is slightly thinner than that of the first layer, and the local scraping area is slightly larger than that of the first layer of putty. Pay attention to the straightness of the edge putty when scraping, and stagger the interface with the upper putty, that is, don't make the interface of each layer putty in the same place to avoid defects, as shown in Figure 5. Should follow the streamline direction (according to the horizontal direction of automobile modeling), and follow the principle of top-down, right-to-left, scraping should be as long as possible to reduce the scraping interface, and pay attention to the putty layer thickness flush with the original coating reference point. Because the scope of the putty layer is gradually expanding, when scraping the second or third putty layer according to the specific situation, adjacent putty layers can be connected together to reduce the edge of the putty layer and facilitate polishing. \ x0d \ x0d \ 7 There are ribs at the lower edge of the door, so special attention should be paid to putty scraping here, and the scraper should be soft. Blow putty with the radian of door reinforcement evenly, as shown in figure 6. \x0d\\x0d\8 The car body is scratched and damaged, often two pieces are not connected together. It should be noted that two pieces should be brushed at the same time in the first pass, and each process should be carried out at the same time in the future, as shown in Figure 7. \x0d\\x0d\9 Polish putty, and polish it after the putty layer is completely dried. For specific drying conditions, please refer to matters needing attention in the use of putty. When polishing putty, please pay attention to dry grinding, not water grinding, because putty has strong water absorption. When the residual water in the water mill can't volatilize well, it will lead to the phenomena of foaming of paint film, "prickly heat", peeling and corrosion of metal substrate. Polishing the putty layer is mainly to obtain a flat and smooth surface. The putty layer can be polished by manual or mechanical dry grinding. Mechanical grinding is suitable for repairing large and flat substrates, which can reduce labor intensity and improve work efficiency, as shown in Figure 8. Manual grinding is suitable for some substrates with complex shapes, such as corners, creases, contours, arcs, recesses, etc. When grinding, the two methods can be combined. \ x0d \ x0d \ 10 The polishing of putty layer basically needs to eliminate all tiny pits and sand holes on the base material, so as to achieve smoothness, no defects, no sand holes and no interface at the edge of local putty scraping, and the appearance diagram is restored as shown in Figure 9. If the radian of the lower rib of the car body door is small, it should be polished by hand, which is beneficial to the correction of the camber. P 120~P240 ~ P240 sandpaper is recommended. Polishing is mainly carried out in the streamlined horizontal direction of the car body. Pay attention to the flatness of the broken line and contour line of the protruding substrate. Generally, there is no need to polish in vertical direction or inclined direction. If the substrate needs to be polished vertically due to specific circumstances, it should be polished and trimmed in the streamlined horizontal direction of the car body to prevent vertical grinding marks. The trace of streamline in the horizontal direction is consistent with the streamline direction of the car body, and their organic combination is not easy to be perceived by the naked eye; Vertical marks are just the opposite, and slight sanding marks will be clearly displayed. For places such as circular arcs and concave corners of the substrate that are not suitable for hand grinding, you can hold sandpaper with your thumb, flatten it with your four fingers, and then grind it evenly back and forth. \ x0d \ x0d \ 1 1 dust removal/cleaning/shielding, use detergent to clean oil stains and stains on the coated object, and use shielding paper. Cover the part of the coated object that does not need to be sprayed with reverse glue, as shown in figure 10. \x0d\\x0d\ Because the air pressure used for primer coating is lower than that used for topcoat coating (in order to minimize spray escape), the shielding process of the workpiece surface is relatively simple. Reverse masking is usually used to prevent the spraying step. The so-called anti-shielding method is that when the shielding paper is attached, the inside faces outwards, so that a thin layer of paint mist is stuck along the boundary. This method is used to minimize the steps and make the boundary less obvious, as shown in figure 1 1. \ x0d \ x0d \ 12 When spraying the intermediate coating, mix the coating and curing agent according to the proportion specified in the instruction, and let it stand for 5- 10 minutes before spraying. If the spraying viscosity needs to be adjusted, a matching thinner should be added, and the construction viscosity should be adjusted to 18 ~ 2 1s (coating -4 cups, 20. \x0d\\x0d\ 13 two-component coating can generally reach the required thickness by spraying 2 layers. If the color hiding power is poor, you need to spray 3-4 layers until it is completely covered, as shown in figure 13. \ x0d \ x0d \ When the surface is partially sprayed with primer, before spraying the first layer of paint, the surface of the coating coated with primer should be sprayed with a thin layer to reduce the color difference between the intermediate primer and the old coating surface and lay a foundation for future spraying. \ x0d \ x0d \ When spraying the first layer, spray a thin layer with spraying pressure of 0.35 ~ 0.45 MPa and moderate humidity, and then continue the construction after checking whether there are any defects such as shrinkage cavity on the coating. \ x0d \ x0d \ After spraying the first layer, if there is no bad condition, let it stand for a period of time before spraying the second layer. The standing time depends on the ambient temperature. Generally, when the coated surface is not stained with dust, it is the best time to spray the second layer of paint. Specifically, you can gently touch the paint film on the sealing part of the adhesive tape with your fingers, and spray the second layer on the wet paint film, as shown in figure 14. Generally, after spraying the second layer, the spraying work is over. This layer of spraying requires the coating surface to be smooth and bright, without flow marks and orange peel. Therefore, when preparing the coating, the viscosity is slightly lower than that of the first coating, the moving speed of the spray gun is also slightly slower, and the spraying pressure can be increased by 0.02 ~ 0.03 MPa, so that the coating particles can be atomized more finely to obtain a satisfactory smooth coating. \ x0d \ x0d \ 14 paint spraying is a very detailed operation, and the use of spray guns should be controlled during operation. If an inexperienced operator can try the spray gun first. After adjusting various parameters (viscosity, air pressure, etc.). ). And then the actual operation, as shown in figure 15. In \ x0d \ x0d \ 15, the paint can be sprayed with a gravity spray gun, which can better control various spraying parameters, as shown in figure 16. The characteristics of gravity spray gun are that it can use low pressure and high flow, and it can fill and cover some small defects on the surface. \ x0d \ x0d \ 16 Pay special attention to the lower edge of the door. There are uneven surfaces such as ribs on the lower edge of the door, so don't pay attention to the defects such as flow jacket and hair, as shown in figure 17. \ x0d \ x0d \ 17 After the coating is dried, the intermediate coating needs to be polished, which can be polished with 400 sandpaper. After cleaning and dust removal, the surface coating can be sprayed, as shown in figure 18. The number of topcoat spraying can be determined according to the coating coverage until it is completely covered. Paint spraying is the last working procedure of painting operation, so all repair shops attach great importance to it and often adopt the method of special personnel operation, as shown in figure 19. Spraying operation also has many problems. If you don't know the spraying tools and operating technical parameters (air pressure, paint viscosity, color, etc.). ) To be precise, try to spray a small sample, and then carry out large-area painting operation after obtaining technical parameters. \ x0d \ x0d \ 18 The door color should be well controlled with the original car body color, as shown in Figure 20, and the coating should not produce color difference after drying. First spray the test piece to adjust the color difference, and then determine the color before spraying the car body in a large area. \ x0d \ x0d \ After baking and cooling the body surface coating. Check and use P 1500 ~ P2000 sandpaper and wet grinding if necessary; If dry grinding is used, use P 1000 ~ P 1200 sandpaper and dry grinding machine; And then polishing treatment can be carried out. \ x0d \ x0d \ After painting is completed, it shall be submitted to inspectors and key personnel for further inspection. The main inspection content is whether there is color difference between the newly repaired coating and the original coating. Whether the repair plane is smooth, whether there are obvious particles and sagging, whether the coating surface is full, and whether there is flying paint on the surfaces of other parts. As shown in figure 2 1, it can be submitted to the customer for acceptance after it has no defects.