Polyurethane is used to coat wood floors to provide a layer of protection against scratches and scuffs.This is an extremely durable wood finish that's simple to use, which has made it increasingly popular. The downside is that if you only use half a can of polyurethane, it can harden and is useless. A few tricks can help you store the leftover polyurethane and prevent it from hardening.
Carefully pop open the can's lid with a can opener or another pry tool that won't bend or damage the lid or rim.
Wipe away all excess finish from the rim and lid before replacing the lid to prevent air from entering the can. If air enters, the polyurethane will harden.
Push the lid into place on top of the can. Place a clean paint rag over the lid to prevent it from splattering. Firmly tap the lid closed with a rubber mallet.
Set the can upside down on the storage shelf. Placing the can upside down prevents air seepage and keeps air and varnish fumes trapped in the completely sealed head space, which is now located above the varnish in the upside-down can.
Add a small quantity of paint thinner to completely cover the top layer of the varnish. Do this if you're uneasy about storing the can while it's inverted or if the lid isn't completely intact. Avoid stirring the paint thinner and allow it to float on top.
Keep the polyurethane varnish stored in a cool location. If it's stored in high temperatures, this can increase the vapor pressure within the can, causing air seepage.