First, make sure your work is fully dry before you varnish it. Oil paint takes a long time to dry, but it really depends on how thick the paint is: A thin layer will by totally dry in 3 months, when a thick application of paint (impasto) may take up to 1 year to be fully dry.
Wipe out any dust with a cloth to clean the surface of your work. This job is easier if you slightly damp the cloth with water. In this case, let the canvas to dry before you apply the varnish, so that you don’t trap moisture between the paint and the layer of varnish.
You will need:
- Some old newspapers to protect the table you are working on.
- A jar or dish to pour the varnish in (the mouth of the bottle of varnish is often too narrow to let you dip the brush directly into the bottle)
- A flat bristle brush (household paint brushes work well for this). It is worth investing in a good quality brush that does not loose any hair and will give you a nice even surface.
To apply the varnish on small and medium works, place them horizontally on a flat surface.
Dip your brush in the dish where you poured the varnish and remove the excess by running the head of the brush on the rim of the dish,
Start at the top of the painting and apply the varnish in straight lines parallel to the larger width of the work. You can go back and forth or go always from the same side to the other side; it does not really matter as long as you keep the lines parallel. Each new line should overlap at the edge with the previous one, so that you don’t have any gap.
When you have finished, start again from the top but this time parallel to the smaller width of your board or canvas. Crossing the initial layer of varnish in this way will ensure an even and complete application.
Impasto can become trouble points because they form ridges that hold too much varnish or they are difficult to cover with varnish because of their uneven surface. One way to solve this potential issue is to start by the area with strong impasto work and scrub the varnish in. Then proceed as described above in order to have a regular application of varnish throughout the whole surface.
The best way to ensure that you have covered the entire work with varnish is to put your eyes at the level of the edge of the painting, facing a light source. This way, the fresh vanish shines in the light and you see immediately the areas you have missed. If you see any drip, pass the brush over them immediately. It is important not to wait, otherwise, the varnish will start to set and the brush will remove the layer of varnish and create an uneven surface.
Checking the varnished surface in the light
You want to “stretch” the application to avoid drips. Once you have checked that all areas are covered, stop and leave the varnish to dry overnight. The varnish should be touch-dry the next day. You can test this by touching the side of the painting with your finger. The surface should not be tacky.
I generally apply only one layer of varnish. If you want to have a ticker application, applying a second layers after the first one is dry is better than trying to put a thick application in one go (with the risk to have drips and cracks).