Topcoat integrity is a crucial property for a protective coating system for protecting metal substrates in conjunction with anti-corrosion primers. Infrared spectroscopy was used to examine the chemical changes seen during accelerated weathering in a model topcoat urethane polymer and to measure the coating ablation. During weathering the ultraviolet absorbance of the urethane coating showed a typical tail (yellowing) into the visible region that increased with exposure period. Effective ultraviolet dosage can be calculated by integrating the spectrum of the incident radiation with the quantum yield for the degradation process and the ultraviolet absorption of the material under investigation. Depending on the form of the quantum yield, there is a clear acceleration of the absorption of damaging radiation because the absorbance increases with exposure. This non-linear relationship offers possibilities on how to estimate a service lifetime; one could choose a value of the exposure period characteristic of the start of the acceleration in dosage, or one might choose the asymptote at which the dosage rate becomes very great. The ultraviolet tailing into the visible region is analyzed as an example of an "Urbach" tail which is usually attributed to structural disorder that introduces energy levels between the principle electronic states.