What are the plants that can be use as an ingredient to varnish?

Shellac is found and processes from Lac beetle shells, hence, shell-lac and it's an interesting process. Some shellac was used a long time ago as a resin component of varnish, and mixed with oils and such. A lot of different resins were used over the hundreds of years, but it is mostly chemicals these days with the exception of some specialty products. Look at the history of shell lac and varnish, it is really interesting, then you will see what goes into formulating it for a special thing; very difficult. For a project, look at buying raw shellac and mixing/ using it. Way cool.