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.]
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