undercoat vs flat coat

undercoat and flat coat both are nouns.

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

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

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

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

undercoat (noun)	flat coat (noun)
thick soft fur lying beneath the longer and coarser guard hair	the first or preliminary coat of paint or size applied to a surface
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

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