A varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent. Today there are also acrylic varnishes.

In application, a varnish is transparent coating applied over artworks to protect against dust, smoke, ultraviolet radiation, abrasion, and other environmental hazards that art is subject to.

Varnish is also commonly used to change the surface sheen and appearance of colors. Some varnishes can also be mixed with artists paints, and for these reasons a varnish can also be considered a painting medium too.

There are many types of varnishes, both synthetic such as Polyurethane Varnishes, and natural such as Shellac Varnish and ones with special attributes that assist the artist, such as Retouch Varnish. 

The ideal varnish can be applied without harming the artwork in any way and protect it without change, however almost all varnishes over time will slightly yellow, and discolor. If fact the most common job of a painting restorer today is to gently remove old varnish and apply a new one to renew the vibrancy of the underlaying color that is still there; protected by the varnish.

Varnishes, for an artists application are available from Artists Paint Manufacturers, or Manufacturers of Specialist Coatings