Many Classical artists knew that if one of their paintings dried too long, they ended up with a surface that was too glossy to absorb new paint. If they had to rework or finish the painting, the new paint would not stick very well to the old paint, making it likely any new work would eventually peel or flake off.
The glossiness of the surface also hindered the movement of the brush, hampering the application of new paint and interfering with brushstrokes.
So what did they do? That’s where the onion comes in.
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Why An Onion?
Onions contain something called ether oil. One of the properties of this oil is that it softens dried paint enough to allow old paint to form a bond with new paint. You can buy onion oil or other types of oils for this purpose, but when onions are so easy to get and so inexpensive, why would you want to?
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Here’s How it Works
So you have a painting that’s unfinished but is several months–or even years old. You want to finish it now. What do you do?
Here’s a painting of mine that’s several years old. I know it’s dry, so the first step is to prepare the surface for new paint.
This painting was painted with a lot of impasto passages and has more texture than I prefer. It also had dirt on the surface, so the first thing I did was lightly scrape it with a razor blade to remove the dirt and knock down some of the texture.
You may or may not need or want to do this, especially if your old painting is also unfinished.
By the way, lightly scraping a painting with a razor blade is a great way to determine whether or not the paint is dry enough for further work. If paint comes off and forms a roll along the blade or if it comes off as shavings, it’s not dry enough. If it comes off as dust, then it’s ready.
Cut a medium-sized or small onion. If you have a larger onion (or a larger hand), you can also slice the onion. I like to leave the outer skins on to help reduce the “tear-making” and to keep the layers of the onion together. A slice of onion works just as well. Just make sure the slice is thick enough to grip and so it doesn’t fall apart.
Rub the onion over the entire canvas. Make sure to cover every part of the canvas. Any areas that are not rubbed with the onion will remain glossy and resistant to new paint, so unless you’re absolutely certain you won’t be repainting an area, it’s best to treat the entire painting.
The next step is conditioning the painting with either linseed oil or walnut oil. I use M. Graham Oil paints exclusively. They are ground with walnut oil as the vehicle, so that’s what I use. Linseed oil also works. You don’t have to match the oil with the type of oil used in your paint, but I prefer that, since it reduces the chances of oils not mixing well.
Pour a small amount of oil into your hand.
Rub it over the canvas. The warmth of your hand and the onion oil will combine to “open the pores” of old paint and allow new paint to form a better, more permanent bond.
You want to use enough oil to cover the surface of the painting, but too much oil will hamper the bond between layers of paint. So after conditioning the painting, wipe off the excess oil with a clean cloth or paper towel.
That’s it! Your old painting is now ready to receive new work.
It’s preferable to condition any oil painting before starting new work. The conditioning process prepares old paint to receive new paint. It also allows brushes to move with greater freedom over the surface of the painting. In that respect, it’s almost like painting wet-into-wet.
BONUS: if you decide you don’t like the new work, you can wipe it off and start over.
And every artist knows that’s sometimes the best tool in the artist’s tool box!
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