Commentary for Avodah Zarah 148:4

Shi'ah - Smooth.

This is a fascinating story for what it tells us about the role of the rabbi in the Jewish community. A man comes to R. Hiyya and wants to essentially kasher a winepress that he probably bought from a non-Jew. R. Hiyya sends out his nephew to makes sure the kashering process is done well, for otherwise people will complain about R. Hiyya in the Bet Midrash. Rav says that smooth parts of the winepress can be koshered by scouring for they absorbed less of the wine that had fermented in there earlier. But the part that had a crack must be treated more stringently. The outer layer of the press must be scraped down. This is still a principle in koshering things like pots, pans, knives, etc. If the surface is smooth, it is less likely to have absorbed taste.