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Pet portraits in oil can be varnished to provide a protective layer that preserves the picture from the atmosphere, it is also used to create a uniform finish whether gloss or matt. Given enough 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.

A varnish should therefore be compatible with the painting it covers but sufficiently different 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.

Before varnishing a painting it is essential 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 high risk of cracking.

Applying the varnish
Successful varnishing should be approached seriously 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 distribute dust particles. Avoid wearing fabrics with loose fibres and cover anything that may shed particles into the atmosphere. To reduce the risk of blooming, the painting and the varnish needs to be in the region of 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 attain a thin, even layer with no brush marks. To achieve this the varnish needs to be the correct consistency, varnish that is too dense should be thinned until it stops dragging on the brush. It’s best to test this prior to application.

Start applying the varnish from top to bottom or top left corner to bottom right, this will help you avoid accidentally 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. Care must be taken in areas of texture or high relief so that the varnish doesn’t puddle, use the tip of the brush to make sure the varnish does not collect in these areas. Varnish will get sticky quite rapidly and any overworking in this state will destroy the smoothness of the surface. A slight drag on the brush will indicate when the varnish is about to turn sticky. When the varnishing is finished, leave the painting where it is and leave the room clear until it has had time to dry.

While a sealed room is the best site for the picture, it isn’t a satisfactory place to be using solvent based varnishes. These give off dangerous fumes and you should take a break once in a while. Also, in no circumstances use a naked flame or heat source in the room, as there is a perpetual risk of fire.