Varnishes provide a transparent coating that protects your finished painting from dirt and dust. Picture varnishes are usually removable, so the painting can be cleaned in the future. Varnishes should not be used as mediums for adding to the colour.
Paintings done in Artisan Water Mixable Oil Colour should not be varnished until thoroughly dry, which takes at least six months. There are three Artisan varnishes available: Gloss, Matt and Satin, depending on your desired finish. The painting will benefit from being degreased before varnishing. This can be done with either Artisan Thinner or Artists’ White Spirit (mineral spirits). Simply wipe it over the surface of the picture sparingly and leave to dry overnight.
Wiping the painting very lightly with white spirit allows you to clean off any grease or grime. If there was any trace of grease on your painting when you varnished it, the varnish would reticulate into beads where the grease was. The cleaning process ensures that the varnish can be applied well.