In order to improve the decoration and weather resistance of the topcoat layer, the metallic glitter color topcoat should be coated on the middle coat, and should not be directly coated on the cathodic electrophoretic primer film based on epoxy resin. For the defects of the topcoat film caused by the defects of the intermediate coating film (particles, sanding lines, dust, etc.) , the metallic flash color is more difficult to repair than the natural color. To eliminate the defects (particles, flow marks, etc.) of the intermediate coating film, the usual method is grinding, and at the same time improve the smoothness of the coating film to improve the adhesion and decorative appearance of the top coat coating film.
There are two ways of grinding, wet grinding and dry grinding. Before the 1980s and 1990s, the grinding of intermediate coatings was mainly wet grinding . Due to the large labor intensity and manual labor of grinding, and it is easy to produce grinding lines (scars), with the advancement of technology, the wet grinding of the intermediate coating film is gradually abolished. The improved examples are listed in the table. Grinding (or semi-dry grinding) is the main method.
The deep sanding pattern cannot be buried when the topcoat is applied, and it becomes an appearance defect. The white color with large pigment particles is not very conspicuous, but the pigment particles such as blue and green are small, and the dense color with low hiding power is easy to be conspicuous. This appearance disadvantage is particularly noticeable when a metallic iridescent finish is applied . As shown in the figure, the aluminum powder can still , and the amount of regular reflection is large, and the normal coating surface is still maintained. In the case of deep grinding scars, because the aluminum powder has a large particle size and is flat and thin, it falls into the groove of the deep grinding scars, and the arrangement direction is disordered. As shown in the figure , the reflected light of the aluminum powder is randomly reflected, and the part with a high angle is dark, resulting in the disadvantage of the "earthworm" appearance of the coating film.
