Does varnish go bad sitting on the shelf?
Unopened varnish should last for years on the shelf when kept away from excessive heat. When a can is opened and the finish exposed to oxygen, curing can begin. If the finish skins over, remove the skin and add mineral spirits to restore the original consistency. Water-based finishes have less tendency to skin over. If they do, add a bit of water to restore the original consistency. It’s always best to test old varnish on a piece of scrap to make sure it’s drying properly before you apply it to a project.
To maximize your varnish’s shelf life after it has been opened, transfer any remaining finish to a smaller container. This will minimize the amount of oxygen in the container and preserve the varnish (see photo, above). Be sure to date and label the container. If you’re using a glass jar to store the old varnish, be sure to keep it out of direct sunlight to prevent damage from UV radiation.
You can also try Bloxygen, a product that is sprayed into a partially used can and forms a gaseous barrier to protect the varnish. It also protects other solvent-based products, such as wood filler and paint, although it will not protect lacquer or water-based finishes. One can is good for approximately 75 applications.