It’s autumn in New England, so I’ve been catching up on varnishing. Why now? Because I have some paintings that have been sitting around for awhile, and because the warm weather is over. That means the humidity is gone. It’s always much better to varnish when the air is very dry. Moisture increases the chances of bloom, in which the varnish becomes slightly milky.
Do you need to varnish?
Yes, you do. Oil paintings need varnish to protect them from being damaged by particulate matter in the air. Varnish also serves to even out the gloss produced by different pigments and to slightly darken the darks, increasing contrast and giving the painting a bit of extra “pop.”
How long to wait
One of the inconvenient things about oil paint is that you need to wait before varnishing. If the paint is applied thinly, you should wait at least a couple of months before varnishing. If you have painted with thicker impasto, it may be a year (or more if the paint is really thick) before the painting should be varnished. If you need to sell a painting before it’s ready for varnish, it’s customary to let the buyer know when you recommend varnishing and to volunteer to do the job yourself if the new owner is willing to ship both ways.
While I don’t like dammar varnish in mediums (it’s too soft and too easy to dissolve if the painting is later cleaned), it’s good for varnishing. You can buy dammar crystals and dissolve them in gum turpentine to make your own varnish, or you can buy varnish pre-made. Never add mineral spirits to dammar, as it is not a strong enough solvent for this purpose. Use only high-quality gum turpentine (if it smells really bad, don’t use it) for thinning dammar. Turps works better for cleanup as well.
Because of the turps, always varnish where there is very good ventilation. Choose a spot that is not dusty. Optimally, you would vacuum the room the day before, then let no one into that room until varnish time, so that the dust can settle and thereby stay out of your painting.
Varnishing smoothly takes a little practice. I’m always scared to varnish a painting I really like, even though it tends to come out just fine. If I haven’t done any varnishing for awhile, I always start by varnishing one or more studies or paintings that came out badly, so I can make any mistakes with them. The first few times you apply varnish, test the whole process out on paintings you don’t care about first. Let those paintings dry and inspect for bloom, missed spots, dust, runs, uneven gloss, or stickiness. If you have such problems, keep practicing on test paintings until you have a process that works consistently for you.
I use a two inch natural hair “gesso” brush for varnishing. A wide brush is a must; with a narrower brush it is too easy to miss a few spots (I’ve made this mistake). If necessary, clean the surface of the painting with a clean, lint-free cloth. You can dampen the cloth slightly with turps if necessary. Make sure that there is no way that water or moisture can get on the painting, the brush, or the container.
When you’re ready, dip the brush in the varnish and wipe off the excess on the lid of the container. Apply the varnish thinly and evenly, stroking the brush from one end of the painting to the next. A little varnish covers a surprisingly large area. Don’t add more varnish to the brush until necessary. I tend to keep stroking back and forth across the painting until the varnish becomes tacky. This tends to reduce drips and also reduce the gloss slightly.
When done, clean the brush in turps, then with soap and water. Make sure the water is nowhere near the newly-varnished painting.
Lean the varnished painting against a wall to reduce the likelihood that dust will land on it. Stay out of the room to limit the amount of dust you kick up until the varnish is dry. In cool dry weather, varnish dries overnight. I’ve heard of it taking much longer in warm, humid weather. That’s one reason why it’s a good idea to test your varnish process on a painting you don’t care about first.
Update 9 November 2008: It’s important to clean your varnishing brush thoroughly afterward. You’ll need to clean it in a strong solvent such as turps. Mineral spirits and other odorless thinners won’t dissolve dammar.