This document describes an automatic painting system for the internal surfaces of solar water heater storage tanks. The system uses an Arduino Uno microcontroller to control a stepper motor that drives a painting gun along a rack and pinion gear system. This allows the painting gun to automatically paint the inside of tanks, improving over the manual painting process. A literature review found that internal tank painting is currently done manually, which is costly and less accurate. The proposed automatic system could help address these issues through controlled, automated painting.