buff vs smoothen

buff is a noun but smoothen is not a noun.

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

As verbs, smoothen is a hypernym of buff; that is, smoothen is a word with a broader meaning than buff:

Other hypernyms of buff include polish, shine, smooth.

buff (noun)	smoothen (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

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

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