Varnish
When considering varnish for an instrument it is important to remember what varnish is there for: first of all it is there to protect the instrument, secondly it is there to enhance the look of an instrument, (bearing in mind the fact that we spend years searching for the incredible timbers used in instrument making), and finally varnish is there to enhance the sound of the instrument. Therefore it is important to create a varnish that is 'permeable,' soft but not too soft, and which enhances an instrument.
These are the principles I try to follow in making my own varnish, and it is also important to remember that 
throughout history there have been violin makers making incredible varnishes all over the Western world – not just in Cremona and Venice!
Personally I use a cooked resin oil varnish using very few ingredients. Throughout the ages there is evidence of possibly 30 to 40 known natural ingredients that have always been used in varnish making, the ones I use are from this list.