(l) phenomenon
The inside of the coating film contains moisture or air and bulges from the surface to be coated or the undercoat, with a diameter of 1-5mm, and those with smaller diameters are called swells. The inside of the coating film contains water and air, and the generation of granular foaming is called foaming. As shown in Figure 9_25.
(2) Reason
Even a good paint film can still be penetrated by moisture. When water vapor penetrates into the paint film, due to the influence of temperature, it may form enough pressure to weaken the adhesion between different coating films or the adhesion of the overall coating to its substrate, resulting in the formation of bubble-like protrusions containing water vapor .
①The surface has been polluted before spraying, especially hydrophilic substances such as sweat stains, fingerprints, saline and alkali, and polishing dust remain on the coated surface.
② Poor water quality for cleaning the surface to be coated.
③The coating film of the paint used has poor water resistance or moisture resistance.
④The coating is not cured enough. Exposure to humid climates or high temperatures before the paintwork is fully dry.
⑤ Insufficient coating thickness of primer and topcoat.
⑥Continuously exposed to high humidity environment, such as in the rainy season, the coating film is prone to blistering.
(3) Preventive measures
①Choose car repair paint with excellent water resistance.
②All surfaces must be clean and pollution-free, and no hydrophilic substances are allowed to remain.
③Pay attention to changing the wet grinding water at any time. All grinding dirt should be removed. When washing with tap water, be sure to wipe and blow dry with a clean cloth, and then dry.
④ Do not touch the painted surface with bare hands.
⑤Painting at the correct temperature, or before painting, the vehicle needs to reach the temperature in the spraying room.
⑥The compressed air should be clean and not polluted, and the compressor and transition should be checked and maintained regularly.
⑦ Spray primer and topcoat should reach the specified thickness.
⑧ Sufficient drying time should be left between each layer, and the coating film should be completely dry.
⑨The coating needs to be fully dried before being exposed to humidity and high temperature environments.
(4) Repair method
Use a needle to pierce the bubbles to determine the depth of the bubbles, and use a low power magnifying glass to find out the cause of the bubbles. When air bubbles occur between coats, the defective area can be sanded away to reveal a good coat before repainting. If the defect is serious, or bubbles occur between the primer and the substrate, all the coating on the substrate should be removed, and then repainted.
