You want to varnish or shellac your project, but you don’t know if the can of finish you have is too old to be good any more.
Here are two things you can do to prevent a too-old finish from ruining your project: 1. Write the date on each can of finish the day you purchase it (TIP: don’t buy any “new” cans of finish that have a thick layer of dust on them; who knows how long they’ve been sitting on the store’s shelf). Some finishes, like shellac, may have a shelf life as short as six months, while others may last a year or two. When in doubt, try the finish on a scrap of wood, to make sure it applies and dries properly.
You can keep finishes from skinning over and oxidizing by filling the empty space in an opened container with an inert gas, such as Bloxygen.