I have a piece done on unstretched canvas that is ready to be finished, so I thought I might take this opportunity to explain a little bit of how I varnish acrylic paintings.
It’s a little more complicated than picking a varnish and brushing it on, especially if it’s a piece you foresee having around for awhile — some varnishes can yellow and craze over time, and may need to be redone in the future. That’s where isolation coating comes in.
An isolation coat is a coat that separates the painting proper from the layer of varnish. From my experience, a layer of gloss medium mixed 50:50 with distilled water works very well for this. Don’t worry about the shiny appearance afterward, though. If you prefer a matte or satin finish, the varnish will take care of that. Do be sure to use distilled water, though. Regular tap water can contain minerals, chlorine, chloramines, and other substances that can damage paintings over time.
Next comes the slightly complicated part: drying and curing. While people generally use the word “dry” to refer to paint that’s finished drying, that’s not entirely correct.
Dry paint is paint that feels dry to the touch. Enough of the moisture has evaporated out of it until you can apply another color over the paint without the layers mixing.
Cured paint is paint that is completely dry. As much of the moisture has evaporated out as possible, leaving the paint as dry as it is going to get.
After the isolation coat dries and cures, it can finally be varnished. The curing part is important — while acrylic media may be dry to the touch in a half hour or so, it may not fully cure for a week. Some people prefer to wait about a month and a half to ensure that the entire painting is fully dry, especially if they’re working with heavy brush strokes and thickly applied paint. Without waiting for paint to properly dry and cure, varnish can end up looking cloudy from moisture trapped between it and the layers of paint.
I like using a matte acrylic varnish with a built in UV protectant. I’m not a fan of sprays, so I generally choose brush-on varieties if at all possible. Some types of varnish, like natural shellac, aren’t that stable and will yellow over time. (This can be more of a feature than a bug, though, particularly if you want to give a piece an aged, vintage look.)
After varnishing, I leave things in a dry, dustless spot to dry. Once the painting is completely dry again, it is ready to display!