Pat B and I were talking about something similar on another thread, so I decided to do some research and this is what I found a post from a guy that makes violins named Micheal Doran I copied his post and pasted it here, I hope this is ok if not please let me know and I will remove it immediately and summarize and re post. You may have to experiment a little but it should lead you down the right road
This is Micheal Doran for the rest of the post
"I have made quite a bit of oil varnish for my violins, so in that area I have some limited expertise. I have never heard of anyone combining shellac and oil, though that doesn't mean it is impossible. I usually, as I'm sure you do, dissolve shellac in alcohol.
To make varnish I combine cooked tree resin and linseed oil, with or without turpentine.
You can use almost any resin to make varnish, from fresh sap to fossilized amber. Every resin imparts different characteristics to the varnish. I use a kind of copal. Copal is a broad category of partially fossilized resin around 40,000 years old, the age makes it copal not what tree it came from.
I cook my resin at high temperature, 395-400 degrees Celsius. This is to burn off the volatile compounds I don't want in my final varnish and to carbonize the resin so it takes on more color. This process of cooking the resin takes four or five hours, and is smelly and a little dangerous. Outside is the only safe place to cook varnish. I usually cook up a big batch of resin because it it easier to control the temperature.
I then combine small amounts of the (cooled) cooked resin to linseed oil. A 1:1 ratio by weight is a good place to start. I then bring that mixture up to 270 degrees celsius. At his temperature something chemically changes in the oil and it combines with the resin and will then dry properly. This is also near the flash point of linseed oil when it can burst into flame. That has never happened to me or anyone I know. I cook it at 270 for 10-20 min then I let it cool and add turps. 1/3 of the volume of the oil is a good place to start, you can always add more to "taste". Sometimes I filter the varnish through a silkscreen cloth, or you can just let some of the jibs settle for a day or so.
I don't know what temperature you need to heat walnut oil up to for varnish, but I can guess it is in that ballpark range of 270 C."