smoothen vs sleek

smoothen is not an adjective while sleek is an adjective.

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

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

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

smoothen (adjective)	sleek (adjective)
	having a smooth, gleaming surface reflecting light; being of a smooth, soft and lustrous quality, resembling silk
	well-groomed and neatly tailored; especially too well-groomed
	designed or arranged to offer the least resistance to fluid flow

smoothen (verb)	sleek (verb)
become smooth	make slick or smooth
make smooth or smoother, as if by rubbing
make (a surface) shine