Elliot Savitsky replies: There are plenty of opinions on the shelf life of shellac, whether you are talking about flakes or flakes dissolved in alcohol. Even if shellac flakes are stored in a cool, dark and dry place, but older than 2 years, it may not dissolve completely in alcohol overnight when compared to fresh stock. I have not experienced that problem, but it’s always best to test out your flakes before committing to a new project. When applied, it may be sticky and slow drying. If used for French polishing, more than the usual amount of oil will be required for lubrication.
Once dissolved in alcohol, shellac can last anywhere from six months to a year and a half. Since I use it frequently for wash coats and do not mix any more than a 2 lb. cut in 1 cup of alcohol, I don’t really approach those limits. I generally can use up my supply within a six month period and my recommendation is not to make any more than you can use during a six month period, keeping it stored in a dark, cool location between 65°F and 70°F. The best way to test the condition of your solution is to put a few drops of shellac on the top of the can of denatured alcohol and let it sit for several hours or overnight. If it’s dry to the touch and you can sand it the next day, it’s good to use.
Bob Couch replies: They generally say two to three years if stored in a cool dark place, but I would lean closer to two years. The great thing about flakes is that you only need to mix up what you need to use in the short-term. I use quite a lot of Zinsser, mostly Sealcoat or Amber and I write the purchase date and open dates on the lid. I buy it in quarts from a local paint and wallpaper store where I know it’s fresher. I also use a shot of Bloxygen in all my finishes when I’m sealing up the can. It’s really just an Argon gas mixture that displaces the oxygen at the surface of the remaining finish in the can. This extends the life of the material and I’m usually able to fully use a can without waste.
The real test of the condition of any shellac is to apply it to a test piece and give it two to four hours of drying time at the proper temperature and humidity, then try to sand it with 220. If it sands to a nice white powder, it’s still good. If it gums up the paper, it should be replaced. You can also pour off what you need for that application and add a few drops of Japan Dryer to the shellac for each application. This will help you get a little longer life out of an older can or mix.