“Gelatin powder (solution) is used in instrument making to treat the wood before staining. It prevents the stain from unevenly penetrating the wood, thus preventing spotting.
Gelatin is mixed in cold water at 5% …” – Excerpt from VIOLIN VARNISHES by Josef and Reiner Hammerl, page 64. More information about linseed oil, its properties and use can be found in this book.