smoothen vs buff

smoothen is not a noun while buff is a noun.

Word	Noun	Adjective	Verb	Adverb
smoothen	No	No	Yes	No
buff	Yes	Yes	Yes	No

As verbs, buff is a hyponym of smoothen; that is, buff is a word with a more specific, narrower meaning than smoothen:

Other hyponyms of smoothen include Simonise, Simonize, sleek, slick, burnish, furbish, gloss.

smoothen (noun)	buff (noun)
	an implement consisting of soft material mounted on a block; used for polishing (as in manicuring)
	a medium to dark tan color
	bare skin
	an ardent follower and admirer
	a soft thick undyed leather from the skins of e.g. buffalo or oxen

smoothen (adjective)	buff (adjective)
	of the yellowish-beige color of buff leather

smoothen (verb)	buff (verb)
become smooth	polish and make shiny
make smooth or smoother, as if by rubbing	strike, beat repeatedly
make (a surface) shine