To smooth or remove small marks that may be left on polymer clay from tools or fingers, we have found that the best method is to use a microfiber cloth dipped in a fast evaporating solvent such as acetone. Wear a mask to protect yourself. You can use it before baking and afterwards too. Acetone can dull micropigment dyes like mica and all metallics.
Also, it is possible to sand the pieces once the clay is fully hardened. It is recommended to use soft grain sandpaper and always work with water to obtain better results. This method is especially useful for cleaning up the back of your earrings or charms.