Oil paintings must be varnished to protect them from undesirable elements such as dirt and toxins. Varnish is a clear solution made from a resin and turpentine or some other solvent.

In art-supply stores you will see two major types, retouching varnish and picture varnish. Depending on the paint layer thickness, it can take six months or more for an oil painting to dry. In the meantime, the surface of the painting needs to be protected with a coat of retouching varnish. After the varnish is applied, the turpentine evaporates, and leaving a thin protective coat. You can apply retouching varnish as soon as the paint feels dry to the touch.