An undercoat is a layer of paint you put on first, before the final colour goes on. Its job is not to look good — it is to seal the surface, hide what was there before, and give the top coat something to grip, so the finished wall looks even instead of patchy. Painters treat it as a real step, not an optional one: 'two undercoats and a topcoat' is normal talk on a job. It is close to 'primer', and people often use the two words as if they were the same, though strictly a primer goes on bare wood or metal first and an undercoat goes between primer and topcoat. In American English 'undercoat' is less common for this and people usually say 'primer'. The word is also a verb — 'we undercoated the door yesterday'. Two other uses are worth knowing. On a car, undercoat (or undercoating, or underseal in Britain) is a thick protective layer sprayed on the underside against rust and stones. And on an animal, the undercoat is the soft short fur underneath the longer outer fur — which is what a dog is shedding in spring when the whole floor turns into hair.

底漆、打底的那層漆（油漆前先上的一層）；也指車底防鏽的塗層，以及動物的底層絨毛。台灣師傅講油漆時多說「底漆」，最後那層叫「面漆」。