If it seems that your painting is little older, evaluate whether or not the paint is good shape but the varnish is fairly aged. In this case, try applying a gentle solvent called conservation liquid. Art supply stores may sell an "emulsion" planned to scrub and take away varnish. There is forever a possibility that the solvent would also damage or get rid of the oil paint. If you are ready to risk this option, wipe the amalgamation with a cotton swab very carefully. Try spot-testing one bend before getting to the whole canvas. Work in a location with sufficient ventilation.
For recent paintings, your problem is more probable a build-up of dirt, smoke, pet hair, dander, and in addition bacterial or any fungal growth. In this case, ensure not one of the paint is prepared ahead off the canvas or board, and thus it doesn't show any cracks or any flakes. Then you could cautiously dust the counter with an extremely spongy and dry bristle brush, such as a baby toothbrush or shaving cream brush.
When the outer lining is muggy, dirty, or oily, you should take the attack a measure further and in fact uses a mild detergent solution. Again, usually speaking, oil and water must never mix, as moisture could without a doubt damage the two canvas and the impasto. Proceeding properly, use fresh fiber cloths dipped in a variety of dish soap and humid water. Lightly spot the outer lining, but do not scrub, clean, or rub in the painting. At no reason should you sink any kind of the painting, nor allow a lot damp that it drips or pools.