(1) phenomenon
Due to grinding, the coating film defect caused by the topcoat coating cannot be covered (Figure 9_l7). For example, the sanding traces shown on the surface of the topcoat are called sandpaper lines. After the topcoat is applied, the abrasive grain expansion becomes thicker and more significantly damages the paint surface, which is called scratches and cracks. There are also scars (caused by sand, tools, etc.) during wet grinding operations called grinding scratches.
(2) Reason analysis
①The quality of the sandpaper is poor, there is a phenomenon of sand loss or use coarse-grained sandpaper.
② The condition of the grinding tool is poor or the operation is not serious.
③ When grinding the plane, no grinding block is used, and the local force is too strong. Figure 9-18 shows the coating film defects caused by different grinding conditions.
A. The bottom layer is rough polished;
B. Grinding before the bottom layer is dry to shrink the coating;
C. Spray the topcoat before the bottom layer is dry, and sand marks will appear on the surface of the coating.
④The last primer before the topcoat is sprayed too thickly, and the drying time left is insufficient. (Related instrument: drying time Tester)
⑤ When the original paint layer is still soft, weathered, insufficiently cured or not completely dry, and is sensitive to the solvent of the new paint, spray the top paint.
⑥ "Scratches" often appear when the topcoat film thickness is insufficient. (Related Instruments: Thickness Gauge)
(3) Preventive measures
① Choose good sandpaper and sandpaper with the correct grade number. Before using the new sandpaper, you should grind it against each other to eliminate the coarse sand grains. Before spraying the metal base paint (the metal base color layer is thin and the covering ability is poor), it should be wet sanded with water sandpaper above 600#.
②The coating before spraying the topcoat should not be too thick and should be fully dried.
③Ensure that the technical condition of the grinding tool is good, and the operation is serious. When grinding the plane, a grinding block should be used, and the grinding direction should be paid attention to.
④ Grinding quality inspection should be carried out after grinding.
(4) Remedy
Depending on the severity of the defect, either sanding or polishing, or wet sanding and polishing with very fine sandpaper, or wet sanding and respraying after the topcoat layer is completely .
