In removing cracked varnish from an old oil painting, gently rub the surface of the painting with the dry fingers. By continual rubbing the varnish will come off in the form of fine dust. Experts sometimes spend days or weeks over a single canvas. Spirit of wine or turpentine may be used to dissolve hard old varnish, but both will attack the paint as well as the varnish if the action is not stopped in time by applying water freely. A weak solution of ammonia or reduced alcohol is also used to soften the surface, which is then slowly scraped away.