Methods of cleaning an oil painting on canvas include brushing the painting's surface with a very soft brush or rubbing the surface with a sliced bread loaf. Both methods help dislodge dust and dirt particles. You can also rub the surface with a soft, damp cloth dabbed with dish detergent. However, conservators warn that these methods can damage a painting. If the painting is valuable, they recommend professional cleaning.
While cleaning a canvas, be careful not to press too hard on it, as this can stretch the canvas irreversibly. Be careful when using soap and water, also. Water seeps through small cracks in a painting's surface and can cause the canvas to stretch and contract, which may lead to paint flaking off. High concentrations of soap can also be harsh enough to damage paint.
Cleaning oil paintings is challenging due to the wide variety of paints and varnishes artists use, leading each painting to react differently to cleaning chemicals. These reactions can harm the painting by making colors duller, removing protective coatings or leaving behind bacterial residue that can eat away at the canvas.
Professional conservators approach each painting individually, sometimes using different methods for separate sections of the canvas. As a result, they are less likely to cause irreparable damage while cleaning.