פירוש על עבודה זרה 120:2

This story is really about kashering the wine press. If the non-Jew is standing in the press then he can be assumed to have touched the wine in the press. Does this wine go into the walls of the press? If there is enough moisture there that were one to moisten one’s hands with it then touch something else the moisture would be transferred then the winepress needs to be rinsed and rubbed dry to make it usable by a Jew. As we shall see on page 74b this process is done with ash. If the winepress is not that moist, then it only needs to be dried off.