From: Rabbi Moshe Sofer
To: Rabbi Lipman
Rabbi Lipman asks how to kasher large industrial whiskey vats — heavily saturated with chametz year-round — for use producing fruit-based whiskey on Pesach. The Chasam Sofer prescribes a multi-step process: thorough cleaning, boiling with concentrated lye, then standard hag'alah.
Source: Shu"t Chasam Sofer OC §120