Waterborne 3-wet paint systems have been developed to reduce volatile organic compounds (VOCs) and CO2 on vehicle painting lines. However, only a small number of vehicles have adopted this system due to limitations in appearance quality (smoothness and gloss). Therefore, a waterborne 3-wet paint system with appearance quality equivalent to a conventional 3-coat 2-bake (3C2B) paint system is under development. This paper describes research for improving appearance quality. After analyzing the unevenness surface formation mechanism of a paint film, this was achieved by adopting base resins with a low glass transition point (Tg) to promote leveling, and reducing the melamine content of the paint to minimize contraction during baking.