We know that varnish needs special brushes and is difficult to clean up if spilled. But storing it is not as difficult if you know how.
To store your varnish, you should pour all leftovers back into the canister. Then make sure you seal the canister securely.
Make sure if there are any drips on the can to wipe them off.
After cleaning up the can you want to store it in a room that is dark and cool. Don't let the varnish get exposed to direct sunlight. Also the temp should be 50 to 80 at the most. Varnish that gets to cold will solidify. And then it is unusable. If it gets to hot it will combust.
Make sure to keep all flammable items away from your varnish as well.