The stability of coil coatings towards exterior influences is one of their most important properties as it ensures substrate quality. The weatherability of eight melamine crosslinked polyester coatings was studied in accelerated aging in order to systematically examine the effect of individual monomers including phthalic anhydride and the bio-based monomers succinic acid, sebacic acid, 1,2-propanediol, 1,3-propanediol and glycerol. Coatings with and without hindered amine light stabilisers were characterised upon degradation through their gloss retention, infrared spectra, film thickness and mu-hardness. A greater susceptibility to radical degradation mechanisms forming new polar chain ends was observed for sebacic acid compared to succinic acid, and for 1,3-propanediol compared to 1,2-propanediol. On the other hand, mu-hardness results suggested that 1,2-propanediol and phthalic anhydride accelerate the formation of new crosslinks during degradation. As radical and hydrolytic bond scission compete with the formation of new crosslinks and each mechanism influences the others, both monomeric composition and overall structure have to be considered to understand the degradation behaviour. The presented findings can be used as a guide by those aiming to increase the bio-based content in coil coatings.