First, a careful evaluation needs to be made of the damage to the painting to see if the dust and dirt is on the outside or embedded in the paint. It is best to do a minimal amount of cleaning. The best initial method is to use compressed air to blow away surface dust. After that, a soft sable brush can be used to lightly brush the surface to dislodge any dust. A small vacuum can also be used to capture the debris. The next level would involve the use of a dry cloth or tape to lift off dirt from the paint. The last resort method to remove soil is to use a moistened cotton swab. Carefully roll it over the surface of the painting, taking care not to change the painting itself while cleaning it.
Mrs. Biddington suggests that artwork is best left untouched except by a professional restorer, since it is easily damaged. However, she offered the following advice for those who feel the need to do it themselves. She suggests buying a loaf or two of doughy bread, preferably sourdough. Put a drop cloth under the painting, pull the bread from inside the loaf and carefully scrub the painting with gentle pressure. The soil collects on the dough and then can be discarded. Do this over the entire surface of the painting. Afterward, use a soft-bristle brush to brush the bread crumbs off of the painting.
According to Gainsborough Products, the safest way to clean oil paintings is by using time-proven museum cleaning formulas. They warn that water-based formulas can seep through the cracks and penetrate the canvas, causing the paint to flake. Wear surgical gloves to protect your hands, and have plenty of ventilation. Oil paintings darken and yellow with age and exposure. Use cotton swabs and roll them gently over the surface of the oil painting. Use a neutralizer, emulsion cleaner and varnish remover for heavy damage. Clean a 2-inch square area at a time. Start in a corner of the painting to see how well it works. After removing dust and dirt, it is best to seal the painting with gloss varnish to protect it from more damage. Most of the cleaning products can be purchased at art supply shops.
If the painting has mold or mildew damage, it can be removed with an antimildew agent that contains sodium hydrochloride. Simply spray some of the agent onto a cotton ball, rub lightly and then blot with a clean rag. Do not leave the agent on the painting for an extended period. Check the cotton ball to see that no paint is being removed.