The painting of the walls was time-consuming and took us two full days.

painting 是动词 paint 的动名词形式。paint 的意思是"油漆、粉刷"，加上-ing 后缀后变成名词，指"粉刷的行为或过程"。 这里 painting 特指"给墙壁刷漆"这项具体工作。