There are a couple of different options for preserving sealant between uses. You could insert a nail in the pierced nozzle, put masking tape or electric tape around the nozzle, squeeze a bit of sealant out and have it harden on the tip of the nozzle, or cover the nozzle with plastic wrap and secure it with a rubber band. All of these methods cover the open cartridge so the sealant inside does not get exposed to air.