undercoat

(noun) seal consisting of a coating of a tar or rubberlike material on the underside of a motor vehicle to retard corrosion. Synonyms: underseal.

(noun) the first or preliminary coat of paint or size applied to a surface. Synonyms: flat coat, ground, primer, priming, primer coat, priming coat.

(noun) thick soft fur lying beneath the longer and coarser guard hair. Synonyms: underfur.

(verb) cover with a primer; apply a primer to. Synonyms: prime, ground.