Ancient artists coated their wall paintings with wax to protect the underlying paint, sharpen colour and add shine. In the Renaissance, the compound of choice for protecting paintings was varnish. Removing varnish is a chemical process between the solvent and the underlying pigments. Though the use of varnish has not been as widespread in the 20th century, its application remains an artistic choice. If you want to rework your varnished oil painting, you will need to use a solvent to remove the resin. Though there are different types of varnishes and solvents, the technique for removal remains the same.
Shoot digital photos of your painting before you begin work. Use these photos as reference points as you make changes to your painting after varnish removal. Take close-up shots of any unique details that require special attention.
Put on your gloves and eye goggles. Lay the dust sheet over your work surface, which should be flat and even. Place the wooden strips over the dust sheet so they fit inside the frame and act as a supporting platform for the painting. This extra support prevents stretching of the canvas.
Remove any surface dirt from the face of the canvas with one of the soft cloths. Dip a cotton swab into your solvent. Test the compound's action by carefully rolling the swab over an inch of the painting, starting at a corner. Do not rub the swab over the surface of your painting in an effort to rush the process.
Look at the swab to check for traces of varnish. If no paint is visible on the swab, continue to slowly roll the solvent over the painting in 1-inch sections. Change the swabs often, so you use fresh materials at each section. Stop work if any paint dissolves.
Brush a thin coat of fresh varnish on the points you want to change and let it dry before repainting. This technique protects the original pigments because you are not repainting directly on them, but on the newly applied varnish. Should your changes not work, you can begin anew by reverting to the new layer of varnish. This is called reversible restoration and is used by professional art restorers.
Photograph the painting after removing varnish and reworking your painting. If you are satisfied with your changes, dry the canvas thoroughly (up to six months) before adding a final coat of varnish to seal the surface of the canvas.
Use the correct solvent for your varnish. For example, Dammar varnish, one of the older varnishes, is best removed using distilled turpentine, which is thicker and more controllable. Other kinds of varnishes, such as those designed specifically with retouching in mind, may use a less-intense solvent for removal.
For museum-quality art or an heirloom with emotional significance, consult a professional restorer. Please note that under normal conditions a painting should not require removal of varnish for routine cleaning. Work near an open window. Most solvents have strong fumes and adequate ventilation of your work space is vital. Also, do not work near a flame or heat source, as the materials are highly flammable. Check with your local sanitation department for proper procedures regarding disposal of your materials (swabs, cloths and so on). Remember that dark colours dissolve quicker than light colours. Take care when removing varnish over darker areas of your painting. Never use soap and water or other household cleansers to remove varnish. Do not use ordinary, household turpentine (such as that used in home decorating) to remove varnish.