If you have never restored an oil painting before, you may wish to take it to a professional painting restorer, particularly if the piece is valuable. If you'd prefer to restore the painting yourself, check to see if it was painted before or after 1940. According to the lifestyle web site Toffsworld (See Reference 1), an oil painting created after 1940 may not have a layer of varnish on it, while pre-1940 paintings are more likely to be coated in varnish. Whether a painting is varnished or not determines your course of action in restoration.
Restore an unvarnished oil painting by simply cleaning it. Remove the centres of about two to three loaves of white bread. Knead this bread so that it becomes "doughy."
Rub the bread very gently across the canvas. This gets rid of built-up dust. The painting should lighten gradually, while the bread becomes dark with accumulated dust. If the painting is soiled by more than just dust, it needs further cleaning.
Dampen a soft cloth and squeeze out excess water. Dab a small amount of dish detergent onto the cloth. Dab the cloth very gently onto the painting -- do not rub it. Start in an inconspicuous part of the painting and work your way over the canvas. Do not allow any excess water to stay on the canvas and stop immediately if you think you are damaging it.
Restore a varnished oil painting by removing the old layer of varnish. Use a solvent that removes varnishes (available in art supply stores). Apply the solvent gently with a cotton swab. Toffsworld (See Reference 1) recommends starting in a corner of the painting and checking for damage before moving on to the rest of the painting.
Add a new layer of varnish to the oil painting with a flat bristle brush. Apply a thin coat of varnish in even strokes. After you're finished, inspect your work. If the varnish appears uneven or insufficient, paint company Winsor & Newton (See Reference 2) recommends waiting 24 hours before applying a second coat of varnish.
Touch up any areas of damaged paint only after you have applied the new layer of varnish. If you make a mistake, this allows you to remove the new paint without interfering with the original painting. You may wish to take photographs of the painting before retouching the paint, to guide you as you work.