Sleek, slick, glossy, velvety, silken, silky, satiny are comparable when they mean having a smooth bright surface or appearance.
Sleek and slick are sometimes interchangeable with this meaning, but more often sleek connotes a smoothness or brightness that is the result of close attention or is an indication, especially when the reference is to a person or animal, of being in excellent physical condition and it may also suggest a smoothness of finish resulting from overattention to or overrefinement in dress and appearance.
Slick , by contrast, is more likely to apply when the intent is to suggest such an extreme of smoothness as to provide an unsafe or slippery surface. In extended use slick is less likely than sleek to suggest desirable qualities and often carries more than a hint of contempt.
Glossy implies a surface that is exceedingly smooth and shining, whether by nature or by art.
Velvety implies the extreme softness associated with the surface or appearance of velvet. The word is often used of things as they appeal to the sense of touch or of sight or of both, but it is also applicable to sounds that caress the ear or to tastes or odors that are delightfully bland.
Silken implies the smoothness and luster as well as the softness of silk. The term is used in reference both to things that appeal to other senses than those of touch or sight and to immaterial things that are softly soothing and pleasant to the spirit or mind.
Silky is sometimes used in place of silken , but when the reference is to persons or their voices, manners, or productions it, more often than silken , suggests an ingratiating or a specious quality.
Satiny applies to what is not only soft but smooth and shining.