(1) phenomenon
During the drying process of the topcoat, the topcoat loses its luster and evenly reflects the defects of the underlying coating film, such as the shape of putty repair blocks and substrate scratches (protruding putty layer, poor edge joint), this phenomenon is called It is sinking color, as shown in Figure 9-15(a). The phenomenon of uneven gloss due to poor grinding (smooth but uneven, or flat but uneven smoothness) during repair painting is called shadow, as shown in Figure 9-15(b).
(2) Reason analysis
① Use sandpaper with large particle size to polish, so that the glossiness of the coating film in the repaired area is quite different from that of the original coating. (Related instrument: gloss meter)
② The base coat is dry-sprayed to form a coarse film foundation.
③ The drying time between the layers is insufficient, resulting in the inconspicuous bite of the primer layer. (Related instrument: drying time Tester)
④ Poor drying conditions—narrow, cold, damp, and no ventilation.
⑤ The primer is sanded before it is completely dry.
⑥ The primer with high pigment content is not stirred every time before use.
⑦Insufficient solvency of the solvent and poor paint mixing.
⑨ Improper trimming of the thin edge of the topcoat layer will also form a dull appearance.
⑩Insufficient topcoat will result in a dull appearance.
(3) Preventive measures
①Use the correct grade-number of sandpaper and carefully grind according to the requirements of the grinding specification, pay special attention to the thin-edged joint grinding technology in the area where the putty is scraped, and it is better to use the developing coating method to test the grinding quality.
② Do not polish before the primer is dry.
③Strictly follow the spraying specifications to carry out spraying operations to ensure that the coating thickness is appropriate and uniform. (Related Instruments: Thickness Gauge)
④ The primer should be stirred effectively before each use.
⑤ There should be a reasonable drying time for each layer. Pay attention to whether the curing agent used has expired.
〇Seal the putty, mud fill and polished parts of all car bodies with primer as needed.
⑦ Improve the drying conditions and increase the temperature of the painting workshop.
(4) Remedy
After allowing the paint to dry effectively, choose remedial measures according to the degree of sinking and shadowing.
① For the shadowed parts on the surface of the coating film, if it is light, use very fine sandpaper or wax to smooth the surface, and then polish; in severe cases, use fine sandpaper to polish and respray.
② For the repaired putty area where the coating film appears, the light one can be remedied by spraying. If there are obvious nozzle connections, topcoat should be sprayed around them to remove the nozzle lines; if the putty block is seriously protruding, the paint film should be eradicated, polished and sprayed again.
