Varnish
A clear protective coating that also deepens colours and evens out the surface shine.
Also: clear coat
Varnish is a transparent layer applied over paint or wood. It protects against dirt and scratches, and it changes how colours look: a glossy varnish makes darks deeper and colours richer, while a matt one softens them. Traditional varnishes were tree resins dissolved in oil or spirit. They yellow with age, which is why many old paintings look brown until a conservator removes the varnish. Modern synthetic varnishes stay clear much longer and can be removed without harming the paint below.