Exploding Shellac: Finally, an Answer
About three weeks ago, I posted a piece concerning a shellac can that blew open as I attempted to pry up the lid using a screw driver. As it turned out, the can I had purchased that morning from a local hardware store was well over four years old.
Ever since that post began collecting comments, I’ve been on the hunt for an answer, contacting various shellac manufacturers in the hopes of getting a definitive answer regarding what caused all that gas and pressure to build up. The folks at Zinsser were kind enough to respond with a rather detailed answer I thought I’d share with all of you.
As it turns out, most of these cans are lined. The slightest flaw in that lining (“even a micro flaw”) can allow the shellac/alcohol mixture to react with the now exposed metal, creating gas and thus, pressure. Zinsser, for one, is aware of the phenomenon and is working out on a solution.
That said, it’s important to understand that the bottom line is, this can was old . . . very old, and while the “maximum shelf life of Bulls Eye Shellac is 3 years,” Zinsser agreed with my recommendation of not keeping shellac longer than one year past the date of manufacture.
So, flip those cans over and check your dates!