It has been a big issue in paint application to maintain steady quality of coatings such as satisfactory appearance without sagging problem through all seasons despite fluctuation of atmospheric conditions. To give a solution for this issue in industrial coating application, we have already launched "Optimally diluted solvent-borne paint supply system". Recently water-borne paints have been adapted in many paint application lines and those paints sometimes require well-controlled air-conditioning in spray booth with huge energy consumption to maintain required qualities. We have developed new paint application system for water-borne paints based on the above solvent-borne paint supply system. In this new system, the original water-borne paint is optimally diluted with an estimated amount of water, and the amount of water is as same as an amount of evaporation under each specific air-condition. The estimation of water evaporation is pre-calculated from difference of water vap! or pressure between that in the atmosphere and on the surface of applied wet paints. This system can secure the required quality of the water-borne coatings by optimization of dilution without the precisely controlled air-conditioning and, thus, the energy consumption for air-condition can be reduced through all seasons.