flat coat vs primer

flat coat and primer both are nouns.

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

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

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

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

flat coat (noun)	primer (noun)
the first or preliminary coat of paint or size applied to a surface	any igniter that is used to initiate the burning of a propellant
	the first or preliminary coat of paint or size applied to a surface
	an introductory textbook