In a nutshell, you varnish an oil painting primarily for three reasons: to equalize the surface sheen, to return the painting's colors to their original intensity, and, to protect the painting against the atmospheric crud which is always floating around. Pollutants which glom onto an oil film and cause it to discolor.
Now obviously, once upon a time, long, long ago, in a world lit only by fire, this was a more pressing concern. Waxy candle smoot and smoky fires could blacken a painting in short order. But even now, with all our modern conveniences and filtered, temperature-controlled environments, it remains a good idea to seal the surface of your work.
The surface of an oil painting may look slick and impermeable, but it is not. A 'dried' oil film is much like an open skin, and rather porous actually, and whatever lands on it can become permanently cross-linked to it over time as the film continues to cure. This isn't good, because to clean the painting and remove that junk would require a solvent capable of dissolving the aged, cured oil that is holding on to it. Which means some of the film itself would have to be sacrificed. Many a masterwork has been ruined by the application of an overly strong solvent, with little or no reason for it to have occurred – if the artist (or the previous conservator) had varnished the work.
You see, a final picture varnish is also a sacrificial layer. It is meant to be stripped away and re-applied every 50 years or so, or as needed. And, if the varnish can be more easily dissolved than the paint film itself, then everything goes as planned. Everybody happy.
So, the ideal picture varnish is a protective coating which levels out the sheen, intensifies the color, won't yellow, is easy to remove, and doesn't cross-link with the underlying oil film over time.
For contemporary oil painters, those requirements reduce the available options down to just a few, which makes our pick easy.
The state-of-the-art conventional wisdom for varnishing an oil paintings is this:
- Use a modern, water-clear synthetic. Old-school damar or copal resin will darken and yellow and cross-link with the paint film over time. To remove it will require a solvent as aggressive as turpentine, if not stronger. A solvent which will be hard on the paint film itself. Or worse, hard on any glazing you might have laid down to modulate a color passage. Again, this has happened to many a masterwork – stripped glazes – which leaves future generations to wonder what the heck the painter meant for us to see. The image at the top of this post shows an old damar or copal varnish being removed. A greatly darkened and yellowed varnish, eh?
- If possible, apply the varnish no sooner than 6 to 12 months after a painting is complete. This allows the paint film to 'cure' enough to minimize potential cross-linking. (Faster drying oils, such as alkyds will permit you to varnish sooner, but my advice still remains as is: wait as long as you can before doing so.)
- Not all synthetics are created equal. Conservators tend to prefer Gamar over others, unless they need to formulate a custom mix for a specific or historical effect. Why? Because Gamar is the closest thing we have to the ideal varnish – off the shelf. It was developed in conjunction with the conservation department at the National Gallery in Washington D.C. It's good stuff.
- Whatever you do, do not use more oil or an alkyd as a final picture varnish. Or, to gloss up a painting for a quick show & tell. The oil you slathered on will dry in time, but will also remain porous, and the extra oil will only compound the yellowing that ALL drying oils must endure. So just don't do it. 'Kay?
Bottom line: Fine, but use as little as you need to perform the task. (I prefer an aerosol-based delivery for this kind of use. Outdoors, or in a well-ventilated space.) Ironically, in this situation you want to use a damar varnish because if the resin will be sandwiched between two or more oil layers then, you want it to be a permanent part of the film! You don't want a conservator to accidentally strip it off, and thus everything that was painted on top of it. Sure, the damar will yellow as time marches on, but if it is applied judiciously to your dark areas, and kept as thin as possible, the resin will have little impact upon the rest of the painting. So in this case, what you shouldn't use for a final picture vanish – damar – is actually what you want to use for your retouch varnish. Go figure.
And finally, do yourself, your collectors, and that future conservator who will be asked to clean your painting a huge favor. Make a few detailed notes on the back of the painting support that reveals the materials used. What varnish was applied, and so on, and perhaps even the order it all went down. If you do it I guarantee that unknown future conservator will light incense in a corner of their shop and add blessings to your name. And then lobby hard to sanctify you in the Great Museum of Archival Painters.
Or, at the very least, your work will have a sporting chance of not being tossed into the nearest landfill...
Next Post: Glossing Over Things: Part II Or, How to Varnish a Painting.