Six different phosphorus-containing reagents were used as self-phosphating agents and acid catalysts in the formulation of a polyester-melamine coating. The phosphorus compounds chosen for the study were Cycat 4040 (4040), Cycat 296-9 (296), Albright & Wilson PA-75 (PA-75), phosphonosuccinic acid (PPSA), and two in situ phosphatizing reagents (ISPR-1 and ISPR-2, phenylphosphonic acid and fosfosal, respectively). Each individual phosphorus-containing compound was evaluated separately using a varying percentage of agent in the paint formulation. The different paints made using the different modifiers were applied to bare 3003 or 3105 aluminum panels. After the individual phosphorus-containing reagents were optimized, the optimized additives were compared to each other utilizing electrochemical impedance spectroscopy (EIS), differential scanning calorimetry (DSC), saltwater immersion, and pencil hardness tests. The most commonly used acid catalyst (4040) did not perform as well as the other acid catalysts. Using 1% phenylphosphonic acid as the acid catalyst and self-phosphating agent in a polyester-melamine coating produced the best protective barrier for the Al substrate of the coatings studied here. Using different acid catalysts in a polyester-melamine paint can dramatically alter the performance of the coating.