Shellac degrade:
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.
Applying a fast drying finish that shrinks substantially over soft undercoats, including over esterified shellac, can cause cracking. In fact, old shellac is sometimes used this way in the motion picture and theater applications to mimic old finishes and paints.
How long is it good?
There is no absolute number, but several factors affect shellac aging. First is the cut. The thicker the shellac is, the slower esterification takes place. Therefore, a five pound cut of shellac will have a much longer shelf life than a two pound cut. How much longer? Depending on other conditions, about eight or nine times longer. In other words, adding alcohol to a can of shellac shortens its shelf life.
Now, about those other conditions; heat, sunlight and moisture are enemies of shellac, even in flake form. Store shellac in a cool, dry place away from sunlight. When shellac resin arrives at the Zinsser factory it is stored on shelves in a huge room that is actually a gigantic refrigerator.
The bad news is that when you buy a can of shellac from the store, you may know how old it is by the date on the can, but you have no way of knowing whether it was properly stored during its travels, its stay at the warehouse, or its life on the shelf. Hence, shelf life remains a moving target and is not an absolute science, at least as far as we consumers are concerned.
What is SealCoat and how is it different?
In 2002 a patent was awarded for a long shelf life shellac resin that substantially retards the esterification process. SealCoat is made from that resin. That means it has a much longer shelf life than other shellac at that particular pound cut. That's important, since the cut still matters. One big difference is that SealCoat is sold in a much thinner, lower pound cut formula, which is possible only because of the extended shelf life resin. Of course, storage conditions still matter also as it will degrade, only more slowly.
Is five year old SealCoat still good?
For most applications, if it was stored perfectly, probably yes. I tested some of the original lab samples five years out and they were fine, but to be fair, my storage conditions are ideal all year around. With different storage conditions, the answer may be different.
How can I tell if mine is still good?
The easiest way is to do a baseline test. Take a known fresh sample of shellac that is cut the same as your questionable sample. Put a drop onto a piece of glass. Check it frequently tracking how much time it takes before it hardens completely, and ascertaining just how hard it gets by pressing a thumbnail into the cured drop. Do the same test with your material and compare both drying times and final hardness.
Incidentally, while it is wise to use only shellac that dries quickly and hard when employing it under other finishes as a sealer, you can use even softened shellac as a final finish. The coating will not be as durable, but it is certainly acceptable. I've seen companies sell lathe finishes that are shellac intentionally softened with the addition of large portions of wax, and these are considered quite appropriate for light use turnings, such as art pieces. As long as it dries in a reasonable amount of time, you can use it; just don't expect the same wear properties as from fresh shellac.
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but several factors affect shellac aging.
