flat coat vs primer coat

flat coat and primer coat both are nouns.

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

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

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

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

flat coat (noun)	primer coat (noun)
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