This study aims to understand the behaviour of a polyester-melamine coating subjected to accelerated artificial photooxidative ageing using analysis at different scales (from molecular to macroscopic) with different techniques (IR, DSC, GC, etc.) and to gain global insight into its degradation. The samples were studied in self-standing films and on coated steel plates with a coating thickness of 25 µm. Two formulations (with and without antioxidant) were characterised, focusing on the degradation of the material under accelerated artificial photoageing. The mechanism of photodegradation of polyester-melamine was investigated by performing different analyses. It was also shown that the stabilising effect of the antioxidant is detected only at the molecular scale (kinetics with IR) but not at the macromolecular or macroscopic scale.