Q. How should I go about removing mildew that's covered an oil painting's surface? And is it possible to clean a painting that appears to be stained with nicotine?

A. If the painting is of value, find a qualified painting conservator to either clean it or give you advice. The reason I recommend this is that the materials used for cleaning paintings can inflict severe damage on the paint layer or canvas.

Mildew can often be removed with a damp swab, and any mold killed by exposing the painting to the sun. A swab dampened with a dilute solution of mild detergent in water can remove nicotine and surface grime. But in inexperienced hands, the moisture from the swab can do severe damage to the varnish, paint and canvas. It can cause varnishes to blanch and turn opaque white.

Loose paint or size in the structure can be damaged and cause paint losses. Plus, canvas often shrinks, allowing the paint to buckle and flake off. For these reasons, applying water to an old painting—not to mention more aggressive solvents, which may be toxic and highly flammable—is best left to the professionals.