The always wonderful Internet Archive has numerous volumes from the day when many people made their own varnish, paint, etc.

Here is a link to books tagged Varnish and Varnishing. If you search the texts section for “varnish” you’ll find even more books.

Some of the volumes lean towards the scientific while others towards the more practical application of knowledge. I think I’ll be doing more experimentation  with mixing and modifying spirit and oil varnish after digging through these a little more.