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Caking: The phenomenon in which a coating forms a skin on the surface of the liquid when stored in a container, as a result of oxidation and polymerization reactions. In the case of oxidatively-dried coatings, factors such as the addition of too much drying agent, a high content of excessively polymerized tung oil, incomplete filling of the coating container, air leakage, excessive temperature, or prolonged storage can all lead to skin formation ； During coating, a piece of kraft paper can be placed on the liquid surface in the storage container after removing the paint; some thinner can be poured onto the paper before sealing the container for storage. Thickening: The phenomenon in which the viscosity of a coating increases over time during storage, making it difficult to apply, is known as thickening. The reason is that the acid value of the coating is too high, causing saponification with the basic pigments ； The tank is leaking air, and the solvent is evaporating ； Moisture has mixed into the bucket, or the paint contains moisture ； Excessively high storage temperatures accelerate the polymerization of the paint ； If the storage temperature is too low, especially with water-based paints, they are prone to cold precipitation. For remedial measures, to the first four types, the appropriate solvent can be added; if water is used, an appropriate amount of butanol can be added. After mixing and adjusting the viscosity, they can then be used. As for the latter type, it can be stored in a warm place for 2–8 days to allow it to thaw and return to its normal state. Precipitation and caking: A defect in colored paints characterized by the precipitation of particles such as pigments and extenders, or the formation of dense lumps that are difficult to redisperse even after stirring. This can be caused by insufficiently fine grinding of pigments and extenders, poor dispersion, or physicochemical reactions between the pigments and the paint base. In coating processes, the precipitate is often fished out, rolled or crushed, then mixed back into the original mixture and stirred evenly before being used after filtration. When coating is applied to a vertical surface, the phenomenon in which the paint flows downward is called sagging. Reasons for sagging include too thin a paint formula with low viscosity and slow drying time; an excessive amount of heavy pigments in the paint or uneven dispersion of these pigments, resulting in poor adhesion to the substrate; applying too much paint onto the brush, which leads to a thick paint film; and high temperatures during application, causing fast-drying paints to dry before they have time to level out. Under normal circumstances, the construction viscosity can be increased as appropriate, the spraying pressure can be adjusted, and the spraying distance can be increased slightly to prevent and reduce sagging. The phenomenon in which the topcoat softens and peels off the film of the undercoat is known as undercoating peeling. The primer has not dried completely, and the solvent in the top coat is too strong ； Issues such as an incompatible primer are all causes of undercutting. Slow drying and re-adhesion refer to the phenomenon where the coating film does not dry within a specified time after application, or where it re-adheres to the surface once it has dried. The solvent has poor volatility, and the topcoat is applied before the primer has fully dried ； The construction humidity is too high, the temperature is too low; the surface treatment is inadequate, with oil, wax, and resin residues, and the wood knots are not sealed ； Impurities mixed into the paint, or the omission of drying agents, curing agents, etc., are all causes of slow drying and re-adhesion. The phenomenon in which the surface of the coating film becomes uneven with tiny particles after the application of granular coating is known as granulation. The reason is that the pigments were not ground finely enough during production, failing to reach the required fineness ； Or aggregation occurs during storage, and the construction environment and tools are not clean ； The paint and solvent are not compatible, resulting in the formation of microgels ； The nozzle has a small diameter, high air pressure, and an excessive spraying distance ； Mechanical impurities are mixed in during material addition, and filtration as well as surface treatment are inadequate. The phenomenon in which a whitened coating turns white and loses its luster after drying, or develops dull spots, is known as whitening. The main reasons are excessive construction humidity (relative humidity > 80%) and rapid solvent evaporation, which cause a sharp drop in the surface temperature of the coating film. This leads to the condensation of water vapor on the coating film, resulting in the precipitation of resin or cellulose and thus the appearance of whitening. Generally, fast-drying volatile coatings (such as nitrocellulose, perchloroethylene, thermoplastic acrylates, etc.) tend to turn white due to the presence of large amounts of low-boiling solvents and diluents. Adding moisture retardants and an appropriate amount of high-boiling-point solvents (such as butanol, butyl esters, pentyl esters) to adjust the solvent formula, or improving the working environment by removing moisture from compressed air, can all prevent whitening. A shellac coating will also turn white when in contact with hot objects, such as a cup containing boiling water; the whiteness can be removed by wiping the affected area with a cotton ball dipped in alcohol or a dilute shellac solution. In the process of applying multi-color lacquers, or as the film dries and forms, unevenities in the color and tone of the coating occur (such as yellowing, discoloration, a golden sheen, and streaks), and this phenomenon is known as mottling. This is mainly caused by differences in the specific gravity and particle size of the pigments, as well as poor lubricity of the pigments, high moisture absorption, high oil absorption, and partial dissolution of certain colors (such as bright red powder). In such cases, this problem can be resolved by adding an appropriate amount of protective adhesive (such as ethyl cellulose, polyvinyl alcohol, etc.) or lubricant (such as methylsilicone oil).