After the sandpaper pattern topcoat is painted and dried, the sandpaper pattern can still be clearly seen and affects the appearance of the coating (gloss, smoothness, fullness and distinctness of image). The streaky scars left by the use of a file on the painted surface are called file lines.
1. Etiology
1) The selected sandpaper is too rough or of poor quality.
2) The surface condition of the object to be coated is poor, with extremely deep filing or grinding lines.
2. Prevention and treatment
1) Grinding sandpaper should be selected according to the process requirements. Before coating the topcoat, use 600~800 water sandpaper for polishing, and use 800~600 water sandpaper for high decorative coatings.
2) Dry the coating and cool it to room temperature before sanding.
3) For occasions requiring decorative properties, wet sanding should be used instead of dry sanding.
4) Improve the surface quality of the object to be coated before painting or fill it with putty.
Insufficient sanding
Painted surface where the surface state of the primer coat is left due to ineffective sanding, which fails to eliminate the defects of the coating.
Grinding and scratching
During the grinding operation, the paint surface is scratched due to abrasives, tools and poor operation.
Grinding pit
Due to improper grinding operation, local grinding has depressions.
All of the above are defects in the coating film caused by ineffective sanding, irregular or sanding scratches, sandpaper lines, and failure of the top coat to cover. Collectively referred to as grinding defects.
Due to grinding defects, the topcoat coating cannot cover and cause coating film defects. After the topcoat is applied, the abrasive grain expands and becomes thicker, and the defect that damages the paint surface is called scratches.
1. Etiology
1) The quality of the sandpaper is poor, there is sand falling off, or coarse sandpaper is used.
2) The grinding tool is in poor condition or improperly operated.
3) No grinding block is used when grinding the plane, and the local force is too strong.
4) The last primer before the topcoat is sprayed too thickly, and the drying time left is insufficient.
5) When the original paint layer is still soft, weathered or insufficiently cured, not completely dry, and sensitive to the solvent of the new paint, spray the top coat on it.
6) "Scratches" will often appear when the topcoat film thickness is insufficient.
2. Prevention
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 loss of coarse sand. Before spraying the metal base paint, it should be sanded with more than 600 water sandpaper, because the metal base color layer is thin and the covering ability is poor.
2) The coating before spraying the topcoat should not be too thick and should be fully dried.
3) Make sure that the grinding tools are in good technical condition and operate carefully. When grinding the plane, use grinding blocks and pay attention to the grinding direction.
4) Grinding quality inspection should be carried out after grinding.
3. 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 respray after the topcoat layer is completely dry.
Putty residue
The part where the putty is scratched on the surface of the coating will produce segment marks or lose light.
1. Etiology
1) Insufficient grinding of putty parts.
2) The putty part is not coated with primer, and the putty layer has a large amount of paint absorption, or the color is different from the primer layer.
3) The putty used has a large shrinkage and will deform after curing.
2. Prevention and treatment
1) Fully polish the puttyed parts.
2) Apply primer to the putty part.
3) Choose putty with low shrinkage. Quick-drying putty such as nitro putty has a large shrinkage and is only suitable for filling defects such as sand holes.
