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
内涂层，底涂层；底漆，底层涂料
Those red walls will probably need two undercoats. 那些红色墙壁可能需要刷两层底漆。
比较
topcoat 返回 undercoat