smoothen vs sandpaper

smoothen is not a noun while sandpaper is a noun.

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

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

Other hyponyms of smoothen include strip, launch, rake, plane, float, sand, sandblast, file.

smoothen (noun)	sandpaper (noun)
	stiff paper coated with powdered emery or sand

smoothen (verb)	sandpaper (verb)
become smooth	rub with sandpaper
make smooth or smoother, as if by rubbing
make (a surface) shine