sleek vs smoothen

sleek is an adjective but smoothen is not an adjective.

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

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

Other hypernyms of sleek include polish, shine, smooth.

sleek (adjective)	smoothen (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

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