Varnish has one basic job: to protect your paintings from taking a beating. And it takes this mission seriously, shielding the surface of a painting from such external dangers as fingerprints, small scrapes and scratches, and hazardous atmospheric conditions including moisture and pollution. Some varnishes are also UV-absorbent, which means they filter out the devastating ultraviolet rays in sunlight. It’s an important job, and as long as the varnish is applied correctly, it’s up to the task. Varnishes are typically thick, heavy fluids made from natural and/or synthetic resins that have been thinned with a solvent. Solubility is one of the characteristics that distinguishes a varnish from a medium.
Varnishes can also affect the appearance of your painting. Among other things, a varnish will determine how much the surface of a painting will reflect light. You may have noticed that, before varnishing, the sheen across the surface of a finished painting can vary considerably. An application of varnish will make the sheen consistent throughout the picture. A gloss varnish will deflect much of the light that strikes the surface, thereby giving it a high sheen. A matte varnish, on the other hand, deflects considerably less light, while a satin finish falls somewhere in between. Colors tend to look deeper and richer behind a gloss. (When a glossy painting is properly lighted, you can avoid the glare that disrupts your viewing of the picture.) Matte varnishes push colors back, and so heavily accumulated applications of it can de-intensify your colors considerably. The choice of finish is strictly a matter of preference and depends upon the effect you want.
Tips for Application
There are a few precautions to take before applying a varnish. First of all, don’t be overanxious to varnish a work. You may wish to delay this step until you’ve had time to resolve any second thoughts about your painting. Touch-up painting on top of a varnish can be quite problematic because of weak paint film adherence. It can also be a problem should a future conservator need to replace the varnish: You don’t want anything that’s integral to the painting to be accidentally removed.
Oil paintings have particular considerations. First, make sure the painting is sufficiently dry before applying the finish. Feeling dry to the touch isn’t enough, it can take as long as a year for an oil painting to dry completely, and if you apply the varnish too soon, it could result in cracking somewhere down the line. Second, be certain that your painting is totally free of moisture, including humidity, before you varnish it. If moisture gets trapped beneath the varnish, you can later have a problem with bloom (a cloudlike discoloration), which can ruin a painting. And take care to use the right kind of varnish for your medium. I once inadvertently applied the wrong kind of varnish to a heat-set oil painting that was almost a year ago, and the varnish still hasn’t dried completely.
No matter what medium you paint in, remember that varnishes can vary from one brand to the next. One manufacturer’s satin, for instance, may look much like another’s matte. Don’t ever try to varnish a work of art if you haven’t tried the product first. Varnishing is like any other skill: It takes practice. Perform your experiments on test samples before you apply the varnish to an actual painting.
Start with a gloss finish and then, if that’s too shiny for you, work your way toward a matte finish until you find the look you want. (The two kinds of varnish are mixable at any ratio you please.) Do your testing over dark colors the darker, the better where the results will be most evident. Once you're certain that you're using the right varnish in the right way, you’ll be able to preserve your works of art for generations to come.