Using these precautions, I have finishes remain useful for a few years. If the finish is in a can (poly, lacquer etc.), just lay a piece of saran wrap over the surface of the remaing finish. This inhibits air contact. When the finish is in the bottle, I top up the bottle to the cap line with shot. When the product is finished, throw the shot in a nylon stocking and soak in some mineral spirits...shot is good to go again.