底漆又称底油，是在髹漆之前预先涂在物料的东西。底漆令油漆能黏附在物料表面、令油漆更耐用、以及为髹漆的物料加上额外保护。

20%-30% 人工树脂、60%-80% 溶剂、以及 2%-5% 添加剂。

A primer (/ˈpraɪmər/) or undercoat is a preparatory coating put on materials before painting. Priming ensures better adhesion of paint to the surface, increases paint durability, and provides additional protection for the material being painted.