From: Rabbi Moshe Sofer
To: Rabbi Yeshayah
The Chasam Sofer categorically forbids the shochtim's practice of smoothing a questionable nick on leather or wood and then declaring the animal kosher, ruling that only a proper whetstone can remove a true nick. Even if the impediment is foreign matter rather than a genuine nick, the effort required to remove it proves it would have caused shehiyah or ikkur during shechitah, rendering the animal treif.
Source: Shu"t Chasam Sofer YD §15