undercoat
noun [ C or U ] uk/ˈʌn.də.kəʊt/ us/ˈʌn.dɚ.koʊt/
内涂层，底涂层;底漆，底层涂料
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
Those red walls will probably need two undercoats. 那些红色墙壁可能需要刷两层底漆。
Compare: topcoat