Varnish consists of three components: drying oil, solvent and resin. The drying oil is a natural plant product; the most commonly used is linseed oil. The solvent that works best with linseed oil is paint thinner. The third component of varnish usually is lacquer. By mixing these three compounds you can make your own varnish. More oil means better exterior protection. More resin makes it better suited for interior use. Solvent dilutes both compounds. You will get less protection but faster drying times.
Pour the linseed oil into a clean gallon can.
Add the lacquer to the linseed oil.
Pour in the thinner.
Mix thoroughly with the flat stick.
There are several different kinds of varnish including marine, musical instrument, shellac and spar. All of them contain slightly different formulas. The quantities and products here are examples. You can substitute tung oil or walnut oil for linseed oil. You can use boiled linseed oil for quicker drying times, or alter the quantities for different results.
Always wear breathing protection and work in a well-ventilated area. Varnish is meant to be brushed on, not sprayed.