Why not put a few coats on a piece of scrap lumber and find out whether it dries properly or not? If it's shot, it's shot -- no point in trying to use it up. If you need to buy more, consider buying a quart can instead of a gallon. Or, order some dry flakes and mix your own as needed.

Hey Jon, thanks for the tip. I did not know that the esterification interfered with proper drying. That gem of info. set me on a search that yielded a longer discussion of shellac esterification, which happened to mention that in 2002 zinsser obtained a patent on a shellac that has a longer shelf life, even when mixed in a thin cut. Turns out this zinsser seal coat is what I was using! No wonder it was still good. Talk about dumb luck! So the technical discussion gives it a 5 year life, zinsser guarantees for three years, so sounds like I'm safe for at least a couple more months.

What I will do is discard the batch I mixed in a small can with more denatured alcohol, and mix up a new batch from the big can. Apparently the thinner the cut, the faster the degradation, so my custom batch would be the first to go.

Now I don't feel like I'm stumbling around in the dark anymore, but I think next time I'll buy the flakes like you suggested so I don't have to worry about this so much. I think the flakes might degrade even more slowly than I progress on my projects :-)

Here is a link that might be useful: how shellac goes bad

If you purchase flakes you can dissolve them on denatured alcohol as needed.

The flakes have a very good shelf life.