How to Varnish a Painting in Oil, Water-Mixable Oil or Acrylic - Jackson's Art Blog
Since artist varnish is meant to be removed in the future when it is dirty and a new coat applied, it is helpful to write in permanent marker on the back edge, which varnish you used. The person cleaning the painting in the future will thank you.
Julie Caves • How to Varnish a Painting in Oil, Water-Mixable Oil or Acrylic - Jackson's Art Blog
marking with charcoal at the moment + taking a note in my references. Use permanent marker instead.