primer coat vs flat coat

primer coat and flat coat both are nouns.

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

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

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

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

primer coat (noun)	flat 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