My canvas attracts quite a lot of dust and cat hair in between painting sessions! Unless I want that embedded into my finished painting, I have to remove it before beginning to paint. The best way is to blast the canvas with some canned air. This removes the loose pieces, and calls attention to the ones partway stuck in the dried paint. These, I have to pull out with my fingers or tweezers.
After the dust and hair are gone, I have to deal with the sunken-in areas of dried paint. Sometimes (or usually!) the oil in a fresh layer of paint will be absorbed into the layer beneath as it dries. This results in a flat, matte look to the new paint, which makes it look paler than when it was fresh. To bring back the shine and correct value, I apply some painting medium onto the canvas with a brush and wipe off the residue so only a very thin layer remains. Now I can I see the painting correctly. Above, you can see what a difference it makes!