Shellac as a sealer? It’s all just hype

You’ll hear shellac tossed around a lot as the “best” sealer, mostly in woodworking magazines targeting amateurs. I’ve come across many professional finishers, however, who believe they should be using shellac rather than the finish itself, a sanding sealer, vinyl sealer or a catalyzed sealer for a first coat.

With only a few exceptions, there’s no reason for anyone to use shellac under another finish. Shellac has been totally overhyped as a sealer. Here’s the story.

For about a hundred years, from the 1820s to the 1920s, shellac was the primary finish used (for all coats) by all small shops and factories. In the 1920s shellac was replaced in factories by lacquer for two primary reasons: shellac resin (from bug secretions) is a commodity product that was going up in price as demand increased, while lacquer was going down in price; and lacquer thinner (a blend of solvents) makes lacquer much more versatile in different weather conditions.

Shellac continued to be used by painters and floor finishers working inside buildings and by amateurs until the 1960s. Then three things happened that almost totally ended shellac being thought of as a complete finish:

Shellac is much more difficult to use (see below) than these three finishes, so it almost disappeared as a finish except in a few niche markets such as French polishing and handmade reproductions of antique furniture.

Companies supplying ready-to-use shellac disappeared one after another until only Zinsser remained. Seeing its market disappearing, Zinsser (Bulls Eye), with the help of some woodworking writers, turned shellac into a sealer, even introducing a dewaxed variety (SealCoat) that was marketed for use under polyurethane.

But here we return to the central question: Why not use polyurethane itself as the sealer? It “seals” the wood perfectly well. Why use shellac under several coats of polyurethane — or under any other finish? The answer is to solve a problem.

Shellac has wonderful blocking properties, better than any other finish. It blocks silicone contamination, which causes fish eye, odors (for example, from smoke or animal urine), and residual wax extremely well.

Shellac also blocks the resin from pine knots and very oily exotic woods, which can slow the drying of lacquer and varnish significantly.

But notice that the first three situations are all refinishing problems, not new-wood problems, and the last is rare for professional finishers.

So for almost all new-wood situations, we come back to asking why use shellac at all?

Not only is there no benefit to using shellac as a sealer in most situations, there are good reasons not to use it. Shellac is a difficult finish (or sealer) to use.

The first reason is the confused naming. Before you even get started, you have to learn the different types of shellac.

In liquid form there are clear (actually pale yellow) and amber shellacs. Until about 20 years ago, when Zinsser changed the names for marketing purposes, these were labeled “white” and “orange.” “Who wants orange furniture?” the Zinsser rep explained to me to justify the name change.

There’s also dewaxed shellac, which is more expensive. Should you be using that? Or will the shellac with its natural wax still included work just as well?

In flake form, which you dissolve yourself in denatured alcohol, there are many more varieties: blonde, superblonde, lemon-yellow, orange, garnet, button, ruby, extra dark and more. These names all refer to the color, ranging from pale yellow to very dark orange.

A second issue is the way solids content is measured. It’s not the standard percentage method used for all other finishes. It’s “pound cut” — the number of pounds of shellac resin dissolved in one gallon of alcohol.

Clear and amber liquid shellacs are three-pound cut. Dewaxed SealCoat is two-pound cut, which is no longer listed on the label. Though conversion to percent solids is possible (so you can predict the total build of your finish), this is another difficulty you have to overcome.

A third issue is shelf life. Once shellac is dissolved in alcohol, it begins deteriorating (more rapidly in hot temperatures). It takes longer to dry and it doesn’t dry as hard. After the shellac has deteriorated a few years in the can, the finish you apply over it may wrinkle.

Shelf life is not a problem if you dissolve your own from flakes (an extra step) because you know when you did this. But it is a problem if you buy already-dissolved shellac. Zinsser has stopped putting the date of manufacture on its cans. So you can’t know how well the shellac you’re using will perform without calling and finding someone who can translate the stamped lot number. You don’t know how long the shellac has been sitting on a store shelf or in a warehouse.

A fourth issue is blushing. You can control blushing with products that thin with lacquer thinner. Just add some retarder. It’s not so easy with shellac because there aren’t retarders available.

A fifth issue is ridging. Unless you thin shellac a good deal, it has a tendency to ridge at the edge of brush strokes and orange peel when sprayed.

If all this isn’t enough to make you question the wisdom of using shellac as a sealer when you don’t have one of the problems mentioned, consider that shellac is a relatively difficult finish to sand. It gums up sandpaper unless applied very thin.

You might conclude from this discussion that I don’t like shellac. This would be wrong. I like shellac a lot.

But my background is refinishing. Shellac is a wonderful tool for solving refinishing problems. It’s also great as a finish when you want to replace an original 19th century finish with the same thing.

But there’s rarely a reason to use shellac in a factory or cabinet shop making cabinets and other objects out of new wood.

Bob Flexner is author of “Understanding Wood Finishing” and “Flexner on Finishing.”

This article originally appeared in the September 2012 issue.