I’ve not always varnished my work. Back when people regularly smoked inside their homes this was a must. The purpose of the varnish is to protect the work and for it to be removable when it starts to yellow from smoke and other environmental contaminates, leaving a perfectly preserved painting underneath. Now, many artists debate on the need to varnish a painting.
Most people don’t smoke inside their homes anymore so the yellowing isn’t an issue most worry about. That’s not to say it can’t yellow given enough time, but it’s just not as big of an issue as it used to be. One major benefit would be smoke damage from a fire. I read an article a few years back where someone who had a very valuable collection of paintings had a fire in part of their house. The paintings were not touched by flame but the smoke turned them black. Because they were varnished, the collector was able to have the varnish removed, revealing perfectly preserved paintings underneath.
For me, a huge benefit to varnishing is that it creates a beautiful even gloss over the whole painting. It’s normal to have areas of a painting where you’ve used more mixing medium end up much more shiny than areas where you used less. Because I work in such light layers, my last layers are always glazes so I generally have an even coverage of shine over the entire painting from my mixing medium, but that isn’t always the case for every painting.
The real challenge with varnishing an oil is that the manufacturers of most varnishes say that the painting needs to be completely dry for SIX MONTHS before applying the varnish. Oil paints don’t actually dry like other paint types, they cure. This process continues for MONTHS (depending on how thick you’ve applied the paint). Those of us who sell our work can not sit on a painting for half a year before we send it to our buyers. If you only sell locally you can offer to varnish later if the customer brings the painting back to you, but that’s not so practical for most artists. This is a huge reason most choose not to varnish their work. There are some you can use sooner but they have to be removed/replaced by a final varnish later on which causes more problems than it solves for most artists.
Recently, I decided I wanted to start varnishing my own work. I went with Gamvar by Gamblin (amazon affiliate link). This varnish can be applied MUCH sooner than other varnishes. Your work only needs to be dry and firm to the touch before you can use it. This product allows the paint to continue curing under the varnish as it normally would. This is a very big deal! Normally, if you put a varnish on a painting that is still curing, it can crack, but this is not the case with Gamvar. The sheen is beautiful with this product! It takes a day to be dry to the touch and two days before you’re safe to ship after applying this product.
A little bit of this goes a very long ways. You don’t need to glob it onto your canvas. As long as you’re applying this in a light layer, you shouldn’t have issues with it streaking. Just remember, you really don’t need to use much!