Restoring unvarnished oil paintings

​Speaking as a studio artist and not a conservation specialist, I varnish my finished art because it makes maintaining the artwork easier for the collector. Glossy varnish can make it more challenging to display and light paintings in certain locations, but I believe my work looks better with varnish, and I’m sure it helps protect against dust and damage from accidental contact.

This is a rather big subject but I will provide a succinct answer and delve further if you require additional information.
Most today look at varnish as an aesthetic choice rather than an automatic method to protect a painting. This view has evolved due to the fact that many works have been inadvertently damaged during varnish removal. Most of this was due to a few prime reasons. 1. The removal was done by an untrained or undertrained restorer. 2. The artist did not follow current notions of best practice and their paint was far more soluble than the restorer believed (see point #1 here as well). 3. The varnish contained oils or other materials that made it far less soluble in “mild” solvents than the restorer believed (again #1 Comes into play here as well).
However, the above does not negate the fact that a reversible varnish applied over a painting that has been allowed to fully dry and one that does not contain extraneous materials (like soft resins) will provide the work some protection. First, it closes up absorbency of the painting. This means that surface grime is either kept as a film over the varnish or is imbibed into the varnish and not the painting proper. We conservators need to approach an unvarnished work in a very different manner than one that has been varnished. But, really a trained conservator should approach every treatment on its own merits. Finally, as Matthew wrote, a varnish does provide some protection from physical damage allowing the work to be gently dusted (with a very soft brush like a badger blender or a goat’s-hair hake brush) without fear of abrasion.

​A collector placed one of my paintings in her kitchen, against a wall on a counter where food was prepared. I noticed little reddish dots in a spray pattern and asked what had happened. The collector wasn’t sure, but then I realized she had been cutting and squeezing lemons near the picture, and the affected areas had quite a lot of ultramarine blue. The painting had actually been varnished, but apparently not heavily enough to act as a barrier.

Some pigments are sensitive to acids. This was a huge issue in 19th-century London where the sulfur dioxide from burning coal permeated the air. This was most destructive of underbound lead white and other acid sensitive pigments. It is also true that acid sensitive marble  sculptures are dissolving in front of our collective eyes.
However, we need to remember that oil paint is an acidic environment. So is egg tempera, so are natural resins. Certainly, they are not as low in PH as SO2, not as deleterious, but it is something to keep in mind. Most Western cities do not have the same levels of pollution as experience in London of the past. Also, even ultramarine is not as sensitive to environmental air pollution as that.
Modern relatively non-yellowing synthetic resins will help the suffusion of these acidic gasses but I’m not sure that the risks are as great as they were in the past. Most artworks are not stored in the conditions described above.
So, when it is said and done, I have to repeat that I believe that a varnish is an aesthetic choice first and foremost. It does confer a degree of protection from some forms of damage and certainly unifies and saturates colors that would likely shift in value and perhaps even hue to a smaller degree if they remained unvarnished, but it is mostly an aesthetic choice.

Interesting to read Matthew. Actual liquid acid, even one as moderate as citric acid could have an effect on a paint film. Lemon juice actually has a PH as low as 2.0 making it rather acidic for a natural, organic acid.

​I think the color shift referred to is vs. the freshly-applied, wet appearance. Colors as mixed on the palette tend to dry with a bit different appearance, especially when the sheen is matte vs. gloss. This is one practical reason why some artists choose to oil out between sessions, so that when work resumes, the previous session work has a similar look to the medium-saturated, fresh paint on the palette. Picture varnish tends to unify dry colors into an overall look that is similar to the wet mixing solutions.