It is better to hang the picture in a bright place, but without direct sun ray. Wipe off dust with a soft, slightly damp cloth or with a dry cloth without detergent!!! An oil painting you cannot hide under glass and store in polyethylene-work needs to "breathe"! If the canvas has sagged a little over time and is hanging in the corners: wipe the picture on the wrong side with a damp cloth and carefully line the wedges with a hammer. It will stretch again! Canvas on cardboard does not require additional stretch and has the ability to be placed in a cheaper baguette than a picture painted on canvas stretched on a stretcher!