27 C.F.R. § 19.384 – Adding denaturants to spirits.

(a) When making denatured spirits, a proprietor must mix the denaturants and spirits only in packages, tanks or bulk conveyances and only on bonded premises. A proprietor must thoroughly mix the denaturants with the spirits to ensure that all of the spirits are effectively denatured.
(b) If a proprietor wishes to use another method of mixing denaturants and spirits not prescribed in this subpart, the proprietor must submit to the appropriate TTB officer a written application for approval of the alternative method in accordance with § 19.27. TTB may require that the proprietor submit additional information, including a flow diagram or other graphic representation of the alternative method, in support of the application.
(26 U.S.C. 5242)