From a site called Riseart...
Removing dust from oil painting canvases
Perhaps your oil painting is collecting dust? The easiest way to remove a thin layer of dust, grime or residue is with a soft cloth and soapy water. Due to its low pH level and mild properties, olive oil-based soap is often considered the most effective soap to use. Make sure you don’t use anything that contains alcohol as this could remove some of the paint. Gently blot your painting with your damp cloth and you’ll instantly see how much dirt comes off. For the corners, or if the painting is particularly textured and impasto, you can always use cotton buds to get into pockets of dirt that have built up over time.