I feel that it is necessary to apply a final varnish to every painting that I complete. I would certainly not even consider showing or selling an oil painting that I had not varnished.
A final varnish for an oil painting should demonstrate these three attributes, at least:
1) It should protect the surface of the painting from dirt, stains, and abrasions.
2) It should impart a relatively even "sheen" across the entire surface of the painting, evening out the high and low (gloss & dull) areas of the painting.
3) It absolutely must be removable, and by the weakest possible of solvents.
Most oil painters observe these first two attributes, but many do not consider the third characteristic.
A finish must be removable because at any time it may be scratched, or damaged while doing its job of protecting the surface of the painting. I once had two paintings stick together, face-to-face, while being shipped in a shipping box. When they were separated, the varnish picked off of each of the paintings, harming the varnish, but not the painting.
All I had to do was to remove the damaged varnish, and re-varnish the paintings. I used Gamsol, one of the weakest of all Odorless Mineral Spirits. I removed the varnish entirely, and WITHOUT dissolving the surface of the painting. Very little paint showed on my cloth, as I removed the varnish.
Natural resin varnishes tend to bond, molecularly, with the ingredients in the surface of the oil painting, and "cross-link" with them. This makes it nearly impossible to remove the varnish, without also removing at least part of the painted surface, as well.
My choice of final varnish is GamVar, by Gamblin. It is a synthetic varnish, and it is easily removable by Gamsol (OMS), without harming the painted surface beneath it.
Synthetic resin varnishes, such as GamVar, can be applied only a few weeks after the painting has become dry to the touch, without waiting the usual 6 to 12 months for "curing time" of the painting. They are more flexible, and they "breathe", allowing the painting to continue drying beneath them, without the varnish forming cracks.
I absolutely never recommend the use of a spray can for applying varnish. It is much, much easier to create an even film of varnish by brushing it on, rather than spraying it on. The only painting that I ever had crack during my years of painting was one on which I had sprayed the varnish. The reason was that I had applied too much varnish by using the spray can.
Once, I also had the bad experience of having a fresh, brand-new, spray-can of varnish spew forth rust, and corruption upon the first touch of the nozzle. I could hardly run fast enough to get Turpentine to wipe the nasty stuff off of my painting. I never used a spray can again, and, as a result, I never recommend spraying varnish.....at least not to my friends.
