While hardier than paints like watercolour and tempera, oil paints come with their own variety of conditions to keep track of in order to maintain the quality of the painting long term. Sunlight in particular tends to be the bane of most paintings, causing the pigments of watercolours in particular to bleach and fade over a matter of a few months, if not weeks, and drastically shortening out the lifespan of a painting, reducing your ability to cherish it and share it with generations of people to come. With oil paintings, it’s much more difficult to determine which of these external elements can cause damage to a painting long-term, and which should not be a concern. This being what it is, a further examination is important – is exposure to daylight a concern for oil paintings, or are they immune to fading from sunlight?
The layering of most oil paintings, which depend on applying gradually thicker and more oil-based layers in many different applications, appears especially well when it comes to natural light. Since, if painted expertly, the natural light penetrates the outer layer of varnish and paint and shine through all of the oil paint, revealing the hours of work gone into layering. You should be happy to know, then, that most oil paint is relatively immune to the fading effect of daylight, only bleaching after many hours of direct exposure to the elements that they are highly unlikely to be exposed to in most interior settings. That is, however, not to say that sunlight can’t damage oil paintings in other ways – the canvas that the oil painting is on can be damaged if exposed directly to sunlight. While the oil paint provides a cover for the most part, the canvas itself should not be put in direct sunlight for fear of long-term damage and decay, so your painting should be examined carefully, and if there are any sections that are visible, they should be put behind , protective cover, and be prepared appropriately so that UV radiation does not cause damage to them.
Similarly, while oil paints are largely immune to exposure to sunlight when they are stored indoors and kept behind glass, there are other effects that are made worse by exposure to light. For example, oil paintings can be greatly damaged if exposed to heat for a long time, with such effects as discoloration, deterioration, and the chemical breakdown of the components that make up the layers of paint.
While sunlight in most places will not bring the paintings to such a high temperature alone, prolonged exposure to direct sunlight in a poorly ventilated place can certainly speed up the process and extend the damage. It is important, when creating oil paintings, to be in a well-ventilated place without any risk of high heat or dangers of chemical accidents, and this also extends to the upkeep of oil paintings. Oil paintings are created using finicky chemical components, which can be highly reactive to heat, poor ventilation, and the exposure to other noxious chemicals, and while this isn’t as instantaneous or as dangerous as when the paint is still wet, it is still beneficial to the long-term health of the painting that these elements be taken into consideration, and that the painting be kept in a safe place with that in mind.
Another aspect to take into consideration, while oil paintings themselves are much hardier than other paintings, is the resin varnish that is frequently applied as the final topcoat on oil paintings. This varnish layer helps shield and protect the painting from damage like things being spilled on it, and gives the oil painting a bright, vibrant shine, it is often more reactive than the oil painting itself, meaning that it can suffer from all kinds of effects that will degrade the quality of the painting. For example, most natural resin varnishes turn to a dark yellow as they age, and the surface of them begins to crack and give the painting beneath a muddy quality, muting the bright colours that the varnish once protected.
With more modern, synthetic varnishes, the yellowing process is much slower, meaning that the painting is protected for far longer. More importantly, if modern varnishes are applied correctly, they can be removed even years later, all without causing any damage to the painting. While sunlight is not the only thing that can cause varnish yellowing, as this is a process that happens naturally regardless of the exterior conditions, sunlight and dust are some of the effects that can accelerate that process even further. I recommend varnishing your paintings to avoid exterior damage regardless. But it is important to consider these risks before committing to a particular varnish, and making sure that the varnish can be safely removed and replaced as part of the long-term health plan of the painting.
In summary, oil paintings are much stronger than watercolour paintings, and do not generally suffer from exterior conditions as much as more fragile paintings do. Conditions like partial exposure to sunlight should offer little to no threat to the quality of the paint, which will only fade in direct sunlight over a very prolonged period of time. However, if the canvas of the oil painting is in any way exposed to sunlight, the UV rays can degrade and damage the integrity of its surface, harming the long-term survivability of the painting in a more indirect way. So it is still recommended that oil paintings not be exposed to sunlight unless their canvas is completely covered.
Likewise, direct sunlight and exposure to high, prolonged heat can damage oil paintings in other ways, such as degrading their chemical makeup and causing discolouration if allowed to go on for a long period of time, as well as increasing the rate at which varnish tarnishes and yellows. This all being the case, you will probably have no problem if you expose your oil painting to some partial sunlight every so often, and it should not be kept in the dark, but I certainly don’t recommend total exposure to sunlight for long periods of time.