The types of dirt or soil to be found on paint surfaces are noted, with particular reference to unvarnished surfaces. The interface between object and dirt is not clearly defined in some cases. Illustrations of this point include: dirt allowed to dry into resin in Duchamp's <i>Large Glass</i> efflorescence from the body of the paint; and the effects of mold growth. Underbound paint films, surfaces which remained tacky after drying, and paints which absorb dirt onto the surface, such as acrylics, present particularly difficult problems of treatment. Examples of unvarnished works which are problematic to clean, all from the 20th century, are discussed.