Things to bear in mind when varnishing an oil painting
Pet portraits in oil can be varnished to offer a protective layer that preserves the picture from the atmosphere, it is also used to create a consistent finish whether gloss or matt. Over time a varnish will itself be attacked, collecting dust and dirt, sometimes cracking and eventually discolouring. Hence, any varnish has to be easily removed and replaced with a fresh coat. The best varnishes are solutions of natural or synthetic resins in spirit as these dry solely by evaporation and can be removed easily using their solvent.
Ideally, a varnish should be compatible with the painting it covers but sufficiently dissimilar so that it’s removal will not effect the paint layers. As a rule-of-thumb, paint media should not be used as a varnish, except where a dilute medium is used as a retouching varnish.
Prior to varnishing a painting, it is necessary that the painting is fully dry. An ordinary oil painting will become stable enough for a light intermediate varnish after six months, and a final thicker layer of varnish should be applied only after at least one year. Otherwise, there is a danger of cracking.
Applying the varnish
Successful varnishing should be approached earnestly if it is to be successful. Initially, the room in which the varnishing will take place needs to be free of dust, and draughts that might circulate dust particles. Try not to wear fabrics with loose fibres and cover anything that may shed particles into the atmosphere. To avoid blooming, the painting and the varnish needs to be around 60 degrees Fahrenheit or 20 degrees Celsius, generally, if both are left in a room at this temperature for an hour prior to varnishing, this should be sufficient.
The painting should be dusted before varnishing with care taken in areas of impasto. The object, when varnishing, is to achieve a thin, even layer with no brush marks. To achieve this the varnish needs to be at the right consistency, varnish that is too thick should be thinned until it stops dragging on the brush. It’s best to test this prior to application.
Start by applying the varnish from top to bottom or top left corner to bottom right, this will help you avoid unintentionally touching the area you have just done. Don’t be tempted to overburden or re-charge the brush, but make sure the varnish you have already laid is spread out thinly. Attention must be paid in areas of texture or high relief so that the varnish doesn’t pool, use the tip of the brush to make sure the varnish does not accumulate in these areas. Varnish will become tacky quite quickly and any overworking in this state will ruin the smoothness of the surface. A slight drag on the brush will indicate when the varnish is about to turn sticky. When varnishing is complete, leave the painting where it is and ideally leave the room empty until it has had time to dry.
While an enclosed room is the best location for the picture, it isn’t a satisfactory place to be using solvent based varnishes. These emit dangerous fumes so you should periodically take time-out whilst working. Also, in no circumstances use a naked flame or heat source in the room, as there is a constant risk of fire.
