undercoat

剑桥词典

a first layer of paint that is put on a surface in order to improve the appearance of the final one, or the paint used for this
內塗層，底塗層；底漆，底層塗料
Those red walls will probably need two undercoats. 那些紅色牆壁可能需要刷兩層底漆。
比較
topcoat 返回 undercoat