undercoat

Short definition from Wiktionary

Noun

1. 1
   A layer of short hairs underneath the longer ones of an animal's fur.
2. 2
   A coat of paint or other material applied onto a surface before that of a topcoat; sometimes, a coloured primer.
3. 3
   A coat for wearing indoors, under an overcoat.

Verb

1. 1
   To apply an undercoat to.