1. Orange peel, grapefruit peel
(l) phenomenon
When spraying, the coating film appears pimple-like wrinkles similar to orange peel and grapefruit peel. The unevenness of the wrinkles is about 3µm, as shown in Figure 9-4.
(2) Reason analysis
The main reason is that the paint coagulates improperly on the surface of the coating film, that is, the leveling time of the coating film is longer than its drying time; the drying speed is different. (Related instrument: drying time Tester)
①The viscosity of the paint is too high, the leveling property is poor, the diluent is improperly selected or the quality is poor. (Related instruments: Sag Tester)
②The construction is not in accordance with the spraying technical specifications, the spraying distance is too far or too close, and the coating is sprayed too thick or too thin. (Related Instruments: Thickness Gauge)
③Low spraying air pressure, excessive paint output and poor Spray Gun performance lead to poor atomization of paint.
④The temperature of the object to be coated and the air is too high, the wind speed in the painting room is too high, and the thinner volatilizes too quickly.
⑤ Short drying time (or insufficient leveling time between coats).
(3) Preventive measures
①Choose a suitable solvent, add a leveling agent or a slow-volatile high-boiling organic solvent to ensure the correct viscosity, so as to improve the leveling property of the coating (the leveling property is compatible with the drying time)
②Adjust the spraying air pressure and paint output, spraying distance and gun speed. Choose a Spray Gun with good atomization performance to make the paint achieve good atomization.
③Spray to the specified thickness once (it should be controlled to the limit of no sag), and the drying time should be extended appropriately, and it is not suitable to enter high-temperature drying too early. (Related instrument: oven)
④The temperature of the object to be coated should be cooled to below 50°C, and the temperature in the spraying room should be maintained at about 20°C.
(4) Remedy
① After the top coat is completely dry and the degree of orange peel wrinkles is small, use fine sandpaper or coarse sand wax to remove the orange peel wrinkles.
② In severe cases, smooth it with fine sandpaper and re-spray.
Two, bite
(1) phenomenon
After the topcoat is sprayed, the undercoat is bitten off, resulting in different degrees of uplift, severe wrinkles, swelling, etc., which are called biting or bottom biting, as shown in Figure 9_5. This phenomenon is easy to occur when spraying a strong solvent-containing paint (such as nitro paint) on the alkyd (oil-based) primer or the original paint surface. Therefore, before spraying, the type of the original coating should be checked (judging the polarity of the paint used in the original car).
(2) Reason analysis
The main reason is that the surface of the coating film is not uniformly cured or the shrinkage rate of each layer is different after curing.
① The coating is not matched, the solvent resistance of the bottom layer is poor, or the topcoat contains a strong solvent that can swell the bottom layer.
②Apply the next layer of paint before the coating is completely dry (in a semi-dry state).
③One time spraying is too thick or the viscosity of topcoat paint is too small.
(3) Prevention and control measures
① After the base coat is dry, apply the top coat.
②Through the test, select the appropriate coating, paint and thinner matching system.
③In the case of matching coatings that are prone to seizure, the first top coat should be sprayed with a thin layer first, and then sprayed after it dries slightly.
(4) Remedy
①In occasions where the defect is relatively light (the primer layer is not involved in the biting), after the coating film is completely dry, effectively remove and polish all the wrinkles and traces, and re-spray.
② When the defect is serious or slow drying due to pollution, it should be removed to the substrate and repainted.
