Things You'll Need
Clean paint rag
Small empty glass jars with lids
Polyurethane finish comes in cans of various sizes. This highly durable wood finish is easy to use, which has helped create its popularity. However, half a can of polyurethane is almost as useless as an empty can. Partially used polyurethane hardens when setting on a basement shelf. Fortunately, there are a few tricks which can help prevent leftover polyurethane finish from drying out and becoming useless.
Open the can carefully with a can opener or a tool which will not distort the can rim or lid.
Clean all excess finish out of the lid and rim before replacing the lid to ensure no air will enter the can as a result of dried, hardened finish.
Press the lid into place on top of the can. Cover the lid with a clean paint rag to prevent spatters and close it completely by tapping it with a rubber mallet.
Place the can upside down on the storage shelf. Inverting the can prevents air seepage and traps air and varnish fumes in the completely sealed head space which is now above the varnish in the upside-down can.
Add just enough paint thinner to the can of polyurethane varnish to completely cover the surface of the varnish. Do not stir the paint thinner into the polyurethane. Let the paint thinner float on top. Use this method if the can lid is completely intact, or if you are uncomfortable with the idea of storing the can of polyurethane inverted.
Store the polyurethane varnish in a cool location. High temperatures increase the vapor pressure within the can which will lead to air seepage.
Store a small amount of leftover polyurethane in baby food jars or resealable glass jars with well-sealing lids. Match the size of an empty, clean glass jar to the amount of polyurethane left at the end of a project. Pour the remaining polyurethane into the jar, wipe the lid to remove any spatters or drips and seal the jar.
Polyurethane varnish cures when it contacts the air and as varnish fumes are allowed to escape from the can. The more air which comes into contact with polyurethane finish, the faster the finish will harden.
Extreme increases in temperature and pressure can cause the varnish to leak from the can if it is stored upside down.