Fitz and I have batted back and forth the shellac going bad issues quite a bit and I hope he chimes in here. What I learned from him was that, yes, indeed, shellac goes “bad,” which is to say that premixed versions (such as Zinsser’s amber shellac, the only premixed version out there) are further into a chemical process called esterification.
To quote a technical discussion
“Shellac degrades. As soon as shellac comes in contact with alcohol, which is its solvent, it begins a degradation process called esterification in which the shellac resins convert to shellac esters. The more esters are created, the slower the shellac dries and the softer and more prone to water spotting the final film is. This process occurs whether the can has been opened or not.”
Now how do you know you have fresh shellac?
That can be tricky too. At my small local owner operated hardware store, the owner told me that he stopped stocking Zinsser’s Amber because demand had dropped off quite a bit. Mind you, he’s speaking from the perspective of a guy who has been operating the store for several generations. He special orders it for me. So popular demand for Amber shellac is quite limited and if the shelf life truly is six months, then the maker has quite a problem, if they have to pull the product back every six months.
What does “more prone to water spotting” mean? My prospector has a shellac bottom and its first coats of shellac have (in some areas) crazed, that is, looks like alligator skin, and tends to white-out (look milky) when water stands on it. I put on a new coat of shellac maybe twice a year which cures the areas that look milky.
If you’re doing this for a customer, and there is a chance the customer will be displeased with the result, I would suggest you buy shellac flakes and mix your own. I think Fitz was experimenting with this and I think it’s what Rollin and Jerry do. Because the coats of shellac you put on will have to be refreshed by the customer in the not distant future, you should alert the customer to the esterification problem.