Rationale: Polymer-based surface coatings in outdoor applications experience accelerated degradation due to exposure to solar radiation, oxygen and atmospheric pollutants. These deleterious agents cause undesirable changes to the polymers aesthetic and mechanical properties reducing its lifetime. The use of antioxidants such as hindered amine light stabilisers (HALS) retard these degradative processes, however, mechanisms for HALS action and polymer degradation are poorly understood. Methods: Detection of the hindered amine light stabiliser (HALS) TINUVIN®123 (bis (1-octyloxy-2,2,6,6-tetramethyl-4-piperidyl) sebacate) and the polymer degradation products directly from a polyester-based coil coating was achieved by liquid extraction surface analysis (LESA) coupled to a triple quadrupole QTRAP® 5500 mass spectrometer. The detection of TINUVIN®123 and melamine was confirmed by the characteristic fragmentation pattern observed in LESA-MS/MS spectra that was identical to that reported for authentic samples. Results: Analysis of an unstabilised coil coating by LESA-MS after exposure to four years of outdoor field testing revealed the presence of melamine (1,3,5-triazine-2,4,6- triamine) as a polymer degradation product at elevated levels. Changes to the coil coatings physical appearance including powder-like deposits on the coatings surface were observed to coincide with melamine deposits and are indicative of the phenomenon known as polymer ‘blooming’. Conclusions: For the first time, in situ detection from a thermoset polymer coating was accomplished without any sample preparation providing advantages over traditional extraction-analysis approaches and some contemporary ambient MS methods. Detection of HALS and polymer degradation products such as melamine provide insight into the mechanisms by which degradation occurs and suggests LESA-MS is a powerful new tool for polymer analysis.