The repeated opening and closing of finish and wood putty cans can leave the lids mangled and unable to form a tight seal. A quick way to restore your can to airtight status is to place a piece of a plastic bag over the lid and then tap it into place. The plastic bag will act as a gasket and help seal the air out.
Note: As your can becomes empty, more air becomes trapped in the can with your finish or wood putty. This air can ruin some products over time. Spraying an inert gas such as "Bloxygen" inside before sealing can prevent this from happening.