1. Differences in the effects of ultraviolet radiation

Natural sunlight: Although sunlight contains ultraviolet (UV) radiation, its spectrum is wider (including UVA, UVB, UVC, as well as visible and infrared light) and its intensity is lower. Mild ultraviolet radiation can assist in oxidative polymerization reactions, while infrared radiation provides heat to promote paint fluidity and uniform curing, forming a smoother and more transparent paint film.

UV lamp: UV lamps typically emit a single band of strong ultraviolet light (such as UVA), which may excessively accelerate surface curing and result in uneven drying speeds inside and outside. This phenomenon of "quick drying on the surface and incomplete solidification on the inner layer" may cause microscopic stress or cracks, affecting the refraction of light and making the paint surface appear dull.

2. Dominance of oxidative polymerization

Traditional oil-based paints (such as linseed oil-based paints) mainly rely on oxidation reactions for curing, and need to combine with oxygen to form cross-linked structures. The heat provided by sunlight can accelerate the oxidation process, while ultraviolet lamps may have limited assistance in oxidation due to their focus on photolysis (if the paint does not contain photoinitiators), and may even damage certain organic structures, resulting in a darker color.

3. Temperature and drying uniformity

When sun dried, the temperature gradually increases and is evenly distributed, which is conducive to the slow shrinkage of the paint layer, reducing internal stress, and forming a dense and transparent film.

UV lamps may overheat locally, causing rapid hardening of the surface layer while the bottom layer remains wet, resulting in uneven curing gradients and affecting transparency.

4. Risk of UV degradation

Excessive ultraviolet radiation may decompose organic components (such as resin or grease) in the paint, causing yellowing or fogging. Natural light has weak degradation effects due to spectral balance, while high-intensity ultraviolet lamps may accelerate this process.

5. Comprehensive impact of environmental factors

Natural sun drying is usually carried out in a well ventilated environment, where humidity and oxygen supply are more suitable for oxidation reactions, while ultraviolet lamps may be used in enclosed spaces, resulting in solvent evaporation or insufficient oxygen exchange, affecting the quality of the paint film.