Having covered working with acrylic paints over the last couple of weeks I felt I should do something similar for oils (watercolours will come later).
When preparing your substrate for oil paintings you need to understand that you are priming the canvas to create a barrier between it and the paint. Just like you do in preparing a canvas for acrylic paintings. The reason for this – and why it is more important with oil paint – is that priming makes the canvas less absorbent. This helps the paint to sit more easily on the canvas and also protects it canvas from natural deterioration.
If you are going to use wood panels (most artists usually choose birch plywood or maple) the same recommendations apply as for acrylic paint. You will need to prepare the board with two coats of GAC100 or gesso to prime the surface. This prevents wood tannins from seeping into the paint, which can cause yellowing. The gesso also helps stop the wood cracking or splitting.
If you are using cotton canvas it is recommended that you use two coats of gesso before you start. If you have an oil primed linen canvas, start with a couple of layers of gesso and then add a layer of oil primer. You can use clear gesso if you want the beauty of the linen canvas to be visible through your paint.
Most canvases come pre coated with gesso and some oil painters I know work straight onto these canvases. Some use acrylic gesso as it is more flexible to use with other media. Most artists I know add more gesso to the canvases as the precoating is very thin. You can add whiting or marble dust to create texture.
Whiting is an inert chalk (Calcium Carbonate) used as a filler for water-based paints and to extend oil colours. It is also used in making traditional grounds for oil painting and in artists’ soft pastels. Grounds are the prepared surfaces, like gesso on a panel or an underlying paint layer on canvas, artists paint on.
This is something my friend Stephanie Thompson does. She will also, if she has a pre-plan of a composition, add an imprimatura layer – thin oil paint, raw sienna, yellow ochre, Venetian red etc – keeping it very pale.
Imprimatura means an initial stain of colour painted onto a white ground.
Once your painting is finished you really need to leave it for at least six months before thinking about varnishing it. If you have used really thin layers of paint, you might get away with less time but if you have thick layers on your canvas, you could easily be looking at a year before it is ready to varnish.
Unlike when using with acrylic paint for your artwork, with oil paintings you do not have an isolation coat, and the oil-based varnish is applied straight onto the oil paint but only when fully dried or cured.