Have you ever found an old painting in a junk shop and wondered if you can paint over it? I have. There are different ways to prepare a used canvas depending if the old paint is oil or acrylic and whether it has been finished with paintable or non-paintable varnish.
It's a time honoured tradition for the starving artist to use or reuse any surface available for their latest masterpiece, so here are some tips on determining the original medium.
1. If you are looking at a painting older than the first world war it is almost certainly oil. Acrylics didn't come into wide use until the latter half of the 20th century.
2. Does your painting have a dingy brown or yellowed look? If the painting is old this probably indicates varnish which is past its due date. Old oil paintings often hung in smokey, industrial, damp or hot areas which yellowed and cracked the varnish. Old varnish must be cleaned or removed before repainting. Sometimes you can strip smoke damage with damp cotton wool, but more often the old varnish needs to be removed completely or you will risk your painting peeling off as it dries and ages.
3. As they age oil paintings dry out and crack. A fine network of cracks all over the painting is a clear indication that you're looking at an oil painting. Dry acrylic paint is rubbery and smooth. As far as we know, acrylic paint doesn't crack with age or yellow with time.
4. Study the texture. Texture is difficult to achieve with unmodified acrylics. Acrylic paint shrinks and flattens as it dries. If gel or modelling paste is added it still doesn't look the same. Oil paint is naturally textured. Crisp brush strokes or thick texture, usually points towards oil paint.
5. Check the material on which the painting was created. Acrylic adheres to raw canvas and any porous or prepped non-porous base like glass. Oil paint degrades raw canvas so is generally used on gesso primed canvas. However, even acrylic artists often prefer primed canvas and some oil painters will paint on raw canvas regardless of the consequences.
6. Observe how the paint blends around the edges of objects. Oil paint dries slowly which allows the artist to go back over and blend. If you find soft edges in acrylic paintings they tend to be slightly stiff and forced. Acrylic paintings are more likely to have crisp edges around objects.
7. Note the quality of the color of the painting. Because oil blends easily the colors in oil paintings can be murky, especially in the top layers. Acrylic paints retain a sharp, vibrant quality.
8. Although oil paint is often blended, if the artist wants to keep the colours separate it is much easier with oil. Acrylics tend to mingle with each other as they dry making it difficult to achieve a crisp colour separation within paint strokes. This is easier to see when the paint strokes are thick and buttery.
9. Think about the clarity of the paint. Oil tends to have a more translucent look allowing light to pass through to different layers below. Acrylics are often matt or opaque, absorbing rather than reflecting the light. Clever varnishing of acrylics can somewhat correct this but that is not common.
More about re-using canvases at another time.
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