The ultimate durability of automotive topcoats is affected by many factors. In recent years, several new methods have been used to study clearcoat degradation including electron spin resonance, hydroperoxide titration, dynamic mechanical spectroscopy, and FT-IR spectroscopy. One important aspect of clearcoat degradation which affects interpretation of the results of all these methods is erosion. Erosion can be caused by a variety of environmental and chemical factors. Measurement of clearcoat erosion has been attempted with limited success by weight loss and FT-IR measurements. This paper will describe an accurate method for determining erosion rates by an optical interference method. This method will be used to demonstrate the effect of light stabilizers on erosion rates.