If we look at oil painting today, many of the practises we accept without question are the result of the limited technology when the oil painting technique was developed. If we look at stretched canvas, still favoured by many artists, we see the only possible way a large flat surface could be produced a few hundred years ago. Small paintings were often painted on wooden panels. These were relatively easy to produce by a skilled carpenter by glueing smaller boards together. Above all they were lightweight. A large panel would be more like the wall of a building and have all the problems of a ‘portable’ wall. Large panels of anything (wallboard, chipboard, cardboard, aluminium, etc) are a recent invention. Large glass panels are even more recent, so framing a large painting under glass was not an option. The solution to protecting the surface of a large painting was varnishing. Like stretched canvas, its still favoured by many artists today.
A hanging painting, not under glass, will eventually need to be cleaned. Varnish will have to be removed and replaced with a clean coating of fresh varnish. The critical factor here is that removing the varnish will not remove a layer of paint with the old varnish. Alkyd resins, like Liquin, are not recommended in the ‘last layer’ of paint applied over traditional oils. These media dry fast and retard the drying of traditional oils underneath so the final layer could detach from the under layers. If you intend to varnish a painting, and you have used an alkyd resin as a medium, the final ‘oiling out’ should be with a vegetable oil. W&N recommend Linseed or their painting medium. It will be necessary to wait several months for this oil to thoroughly harden.
I use a wide brush to apply varnish. The problem here is cleaning the brush after varnishing. If every trace of varnish is not removed, the brush will harden and cannot be used again. It would take an enormous amount of good quality spirits to clean a brush of 2″ or 3″ width. I have solved this problem by suspending the brush by a wire contraption in an airtight glass jar (see photo), with a layer of spirits at the bottom of the jar. This stops the brush from drying out, so I never have to clean it and it is always ready to varnish.
Apply the varnish gently with horizontal then vertical overlapping strokes. Gently to avoid bubbles in the varnish and also disturbing the paint layer. Dust is a problem both applying the varnish and waiting for the varnish to dry (lie the painting flat) so vacuum the area before you start. Don’t use a sweeping brush as this will only make the dust airborne to later settle on the painting surface.
The range of varnishes available is easy to check out, online or in an art suppliers shop. Windsor & Newton have a gloss and matt varnish which can also be mixed to produce an in-between finish (like satin) to suit your personal preference.
If you very few paintings to varnish, or you only varnish occasionally, you can use the aerosol varnish. This is fast and easy to apply but costly.
All this bother with varnish is necessary if the painting is not framed under glass. I frame most of my paintings under glass, after ‘oiling out’ with Liquin. This is more to do with the time involved with varnishing than anything else. At some time in the future the painting could be ‘oiled out’ and varnished because, hopefully, by then the painting would be thoroughly dry and clean.