"Traditional" gunlock varnish was a shellac based preparation , Took a bit of practice to apply as you could only make one stroke to cover an area as its quick drying made it difficult to go over a second time . The main purpose is to protect the colour hardening as well as to give it shine .A spirit base makes it easy to remove and l like to re-do it when ever a gun is fully stripped and cleaned . There are other lacquers and varnishes that have been used to good effect including commercially available sprays . All do the job but great care must be taken to ensure you do not get it where it is not wanted .