The coating behavior of a polyester-melamine paint catalyzed by an in situ phosphatizing reagent (ISPR) was investigated, and the results were compared to p-toluenesulfonic acid- (p-TSA-) catalyzed paints. Fourier transform infrared spectroscopy was used to ascertain that the reaction between the ISPR and the Al surface takes place. Differential scanning calorimetry was used to measure the glass-transition temperature (T-g) and the T-g thus providing an indication that the ISPR catalyzed the paint film formation. The deterioration of the coatings by overbaking was studied using thermogravimetric analysis. The corrosion resistance of the coatings on aluminum coupons was examined using electrochemical impedance spectroscopy. Equivalent electrical circuits were utilized to give numerical values to the resistors and capacitors in the coated panel. The paint film adhesion was tested by saltwater immersion. The high-quality performance of the ISPR-catalyzed paint can be attributed to the simultaneous reactions of the paint curing with the acid provided by the ISPR and the in situ phosphatization of the metal surface. Using the ISPR as the catalyst instead of p-TSA gives rise to a more corrosion-resistant film and a more thermally stable coating.