According to the coating equipment in the workshop and the state of the painted bus body, the bus coating process can be divided into two coating systems: high-temperature drying and low-temperature drying (or self-drying). The former selects the car production line (OEM) series of automotive baking coatings, which are suitable for painting workshops with drying equipment and body coatings that can withstand high temperature (12~140°C) drying. The latter uses paint for car repair, generally a two-component series paint, suitable for occasions without high-temperature drying conditions and body bodies that cannot withstand high-temperature drying conditions (such as those installed on the chassis, that is, with tires and engine assemblies). Body or body structure , the body will be deformed after high temperature drying) coating. During the execution of the process, the following process points should be paid attention to.
① Putty and mid-coat. In the process of mass production and high-quality body painting, the primer layer is sprayed directly on the primer layer (or if necessary, check the rice gold modification and sand it with fine gauze paper before the mid-coat). However, in the production of passenger cars in our country, the mold is poor, the quality and assembly of the sheet metal parts are poor, and it is inevitable to putty. Scraping putty only improves the appearance and decoration , but is harmful to other properties of the coating, so it should be scraped as little as possible. The commercially available putty (unsaturated polyester putty ) and water-based putty have better construction performance than traditional gypsum putty, oily gray putty and epoxy putty, so they are widely used in bus painting. Atomic putty has a fast curing speed, and it will dry and hard within tens of minutes or even a few minutes after scraping. It can be polished, has good adhesion to steel plates, good mechanical strength, and is not easy to crack and fall off. It is a low-temperature curing (self-drying) coating system. Comparable putty. Only works well with dry cathodic electrocoat and epoxy primer (two-component or baking type), if the primer is of poor quality or not fully dried, the putty will easily bite the primer and peel it off the steel plate down. In order to reduce the volume shrinkage of the putty layer during the curing process , although the putty can be applied thickly, it should be applied and scraped several times. The unit price of putty is expensive, and the grinding allowance should be kept as little as possible.
Water-based putty consists of latex, fillers and additives. With the volatilization of water after coating and scratching, the latex particles are bonded to form a film under the action , and the high-temperature baking promotes the volatilization of water, softening the latex particles, and making them more tightly bonded, resulting in better physical and mechanical properties. Generally, it is better to dry at 110~140°C. If the drying temperature is too high, the polymer will decompose, and the water will not volatilize completely . Water-based putty is suitable for high-temperature drying coating systems.
The advantages of water-based putty are cheap, easy construction, easy grinding, and low toxicity. Compared with epoxy putty and putty, the putty layer is loose. A better construction method is to apply and scrape several times (thin and multiple layers), each layer is dried and polished, and then the primer is applied to penetrate and strengthen the putty layer. Attention should also be paid to the matching with the topcoat, because the water-based putty layer is easy to cause air bubbles in the topcoat layer. In order to seal the base coat, create a smooth and well-bonded base coat for the top coat , enhance the brightness, sharpness and fullness of the top coat layer; use the "wet-on-wet" process to spray a layer of dry film thickness of 35~45μm middle coat. Use methods such as scraping putty and fine paper wet sanding to eliminate the coating defects that appear in the middle coating. For the quick-drying putty used for the trachoma and many paper textures in the plugging, acrylic should be used faster than the putty. Nitro-based putty should not be used, as it will still have spot marks after shrinking for a long time. Spray anti-sound and anti-crack damping paint on the inner surface of the body after primer or intermediate coating to improve the comfort and corrosion resistance of the body.
②Finish paint and color strip coating process. Due to the large area of the passenger car, it is monotonous to paint one color, usually more than two colors, so the topcoat layer can be subdivided into two types: basic main color and color stripe color. Generally, the main color topcoat is sprayed first, and then the color strip color topcoat . If the two colors are close together or close, you need to spray one color strip to dry before spraying another color strip. If there are 4 to 5 colors, it needs 2 to 3 times, which is time-consuming and energy-consuming.
In order to save energy and improve work efficiency, in the high-temperature drying coating system, the main color topcoat can be used as a baking type, and the color strip color topcoat can be used as a two-component drying topcoat; some use pasted color strips and signs . In the case of spraying flash color topcoat and using basecoat and varnish process, you can spray the main color basecoat and dry it to the surface, then spray the color basecoat, and finally clean the entire surface and spray varnish After varnishing, dry in one coat.
