It is forbidden to smooth out a solid (or semi-solid) object on Shabbat. One of the 39 forbidden acts on Shabbat is "memachek" which means smoothing out a hide by removing all its hair.

This is also why it is forbidden to apply creams on Shabbat.

[Incidentally, this rule does not apply to food. It is, for example, permitted to spread butter on bread on Shabbat.]