There are many benefits to painting with oils, especially when it comes to the hardiness of oil paints once they’ve dried completely and been allowed to cure. Even when unsealed, oil paints are chemically more adept to dealing with sunlight and other contaminants that can cause damage to other paintings. However, it is customary, even for oil paintings that have been around since the Renaissance, to apply a layer of varnish over the topcoat of an oil painting. This process protects the integrity of the painting, and increases the vibrant colours of the artwork, capturing light in a beautiful way while also maintaining the texture of the original impasto beneath.
However, varnish is also tricky to work with, and is often more reactive than the paint itself, inevitably yellowing with time and darkening the painting, a process seen frequently in the older artworks that remain iconic in media, even as their details become lost to time. This being the case, is it truly always necessary to varnish a completed oil painting?
To answer the question simply: you do not have to varnish all oil paintings, but you almost always should.
And here is why.
Oil paintings without varnish will still dry and cure properly, and the oil painting itself should still be relatively sturdy. Since oil paint, if allowed to dry properly, is a very stable and protective paint that can tolerate many things without suffering outwardly. However, just because you can leave an oil painting unvarnished without immediate damage showing through doesn’t mean that you should do so. Especially since oil paints, without varnish, tend to dry at different rates, which means that, as they reach their final stages of curing, they often give off an uneven, somewhat patchy appearance, with some colours drying to a more matte finish and others a more glossy one. Adding a layer of varnish, regardless of your method, allows for a more even surface shine, which can be controlled using different methods, all while protecting the integrity of the surface of your painting for years to come.
Part of the aversion to adding a varnish glazing to all oil paintings comes from the kinds of varnish we often see in museums, which is the yellow, chipped variety often seen on very old, very fragile oil paintings that have survived centuries with the same coat of natural resin. However, the fact that we can see those paintings at all is an indication that the varnish, through much yellowed and often difficult to remove, has done its work.
All varnish, even modern coats which tarnish at a much slower rate, yellow to some degree over time, as a function of their task of protecting the painting from prolonged exposure to sunlight and capturing contaminates like dust, smoke, and other chemical damage. This leaves the vanish yellowed and cracked, and, in more fragile paintings which were made before more modern varnishing techniques were widely available, very difficult to remove without causing lasting damage to the oil paint beneath. These days, however, varnish is almost entirely artificial, and can be applied through brush as well as using aerosol, creating thin layers that are much easier to remove using solvents, and carry little to no risk of damaging the painting when they are replaced with other, newer coats of varnish. Like means that modern varnish acts much like a dust jacket – it protects the integrity of the painting, while remaining entirely replaceable once its purpose has been completed.
Another aspect of varnish that goes underappreciated is that it tends to “revive” colours that have dried, especially when it comes to oil paints, which are painted on in colours that do not reflect what they will look like when dry. Wet oil paints, whether saturated “lean” with turpentine and paint thinners, or “fat” with various oils, have a very different colour when brushed on, still damp, to what they look like hours or even days later when they have cured. The layer of varnish, which can look very much like a coating of oil, adds the effect of light and glisten to the dried paint, returning to it a lot of the vibrancy that is lost through the curing process. The result, if completed correctly, should be that the painting not only looks as good as it did without the varnish, but, in fact, closer to the original intent.
Some artists’ distrust of varnish comes from the lack of texture that it presents, especially when painting impasto, or when trying to give the brush strokes a tangible visibility when viewed in a gallery. Most varnish tends to flatten out a painting, removing the play of light on the various angles and shapes created in the paint. However, using aerosol varnish, which is much thinner, these textures can be captured far more accurately, and applying a slightly rough texture to the final coat of gloss using a rag, a rough brush, or anything else that works, can give the painting far more character, allowing for protection and even shine that does not seem artificial.
Lastly, it is important to use varnish on oil paintings whenever possible because you do not know the future of your painting when you finish it. We hope that our paintings will be presented under the best circumstances for their entire duration in this world, but circumstances change over the years – many paintings without varnish have suffered from smoke damage, either from the places they are being stored suffering fires, or through the prolonged years of cigarette smoking in their presence. Standards of art conservations have changed rapidly over the last century, and will likely continue to change as time goes on. While using reversible varnishes and glazes does not make your art immune to the march of time and the changing expectations of art, it gives your painting the chance to be presented, and protects it from all kinds of dangers, both the ones we know about now, and the ones we will discover in the future. Investing in varnishing acts as insurance for your painting, and gives it a far greater chance of lasting as intended, even centuries down the line.