I've been working on several vintage flutes recently that suffer from the same issue: a crumbling, whitish bore filler. We often call this Seko-ji  (plaster-based ground). It was used as a faster alternative to the traditional Tonoko powder & Urushi mix. I was curious about the history of this practice, so I pulled a book from my shelf: 'Shakuhachi Making'  by Kamizuki Enzan . The original text dates back to 1933 (Showa 8) . In the book, the author explicitly writes: “I use '