There has been much debate over whether artists should varnish their oil painitngs, and I think the word is still out, depending on who you ask.  A varnish is a resin applied to the surface layer of an oil painting. It adds sheen and can protect the painting from dust and pollutants. The varnish can be removed by conservators for cleaning without removing the paint layers underneath. But varnish also has yellowing properties and can turn quite dark over time. It can also crack the paint beneath it if it is applied when the painting isn't completely dry through and through. I have also seen paintings ruined by varnish that was applied too thickly or unevenly, so you have to be careful with it. 

In earlier times, varnish could protect from the dirt particles put out by burning coal, etc. But the Impressionists did not varnish their paintings because they wanted to avoid the yellowing properties perhaps, but mainly because they preferred a matte look to their paintings (if you go into a museum today you may see varnish on the surfaces of many of these paintings, but likely they were not put there by the painters themselves. Interestingly, Monet in particular preferred a very matte look and white, plain frames for his paintings; not the heavy ornate gold ones you see in the museums.)

Today varnish seems to be purely a optional decision, and mainly an aesthetic one. The Gamblin website has some good info on the topic of varnishing here. If an artist likes the look of a varnish, she can apply one, but only after the painting is completely dry. Oil paintings that have any thickness at all generally take about 6 months to dry through and through. If not, serious cracking can occur because the top layer of varnish will dry faster than the layers beneath.

In all honesty many of my paintings don't hang around in my studio for 6 months. What I usually do is to apply a retouch varnish once my paintings are dry to the touch on the surface. A retouch varnish is so lightweight that it becomes part of the top paint layer, so you don't risk the cracking that a heavier varnish can do. A retouch varnish can also even out the surface of the painting, bringing out darker areas that may have "sunken" to a more lustruous appearance. The effect is to provide a lusterous protective sheen to the painting, which I prefer to a super shiny surface.

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