flat coat vs undercoat

flat coat and undercoat both are nouns.

Word	Noun	Adjective	Verb	Adverb
flat coat	Yes	No	No	No
undercoat	Yes	No	Yes	No

As nouns, flat coat and undercoat are synonyms defined as:

flat coat and undercoat: the first or preliminary coat of paint or size applied to a surface

Other synonyms of flat coat include ground, primer, primer coat, priming, priming coat.

flat coat (noun)	undercoat (noun)
the first or preliminary coat of paint or size applied to a surface	thick soft fur lying beneath the longer and coarser guard hair
	the first or preliminary coat of paint or size applied to a surface
	seal consisting of a coating of a tar or rubberlike material on the underside of a motor vehicle to retard corrosion

flat coat (verb)	undercoat (verb)
	cover with a primer; apply a primer to