Pros: easy to apply; dries quickly; non-toxic once it dries; made grain more uniform in color
Cons: lengthens drying time of stain applied over it; apparently doesn't repel water that well
I had never used Zinsser Bulls Eye Shellac until recently when I refinished a pair of plywood Acoustic Research AR-3a loudspeakers. Plywood can look pretty ugly, even when stained, so I consulted with a wood working friend about how to proceed. He told me that a thin coat of shellac, diluted four parts to one with denatured alcohol, was the way to go as a base before staining. He said that the shellac would be absorbed by the wood that was likely to accept too much stain, and that the result would be a more uniform look when I applied the stain to the plywood surfaces.
Zinsser Bulls Eys Shallac appears to be something of an all-purpose protective sealant. Although it's known primarily as a wood-sealing agent, it says on the can that you can use it on tools, golf clubs, wood toys, and even ceramic tile. It is said to dry within a half hour, and to be ready for additional coats in an hour. It is supposed to dry to a "mid-gloss sheen" and to block odors from older wood, plaster drywall and masonry.
Shellac is actually made from the secretion of a tiny insect (Laccifer lacca) prevalent in Asia (especially Thailand and India, according to one source) and has been used as a protective agent for many centuries. Although liquid shellac is generally suspended in a flammable and potentially toxic medium (in this case, denatured alcohol, I assume), the product is non-toxic once it dries, so you can use it on toys likely to be nibbled on by young children.
It turns out I was a bit of a rebel in my use of Zinsser Bulls Eye Shellac. On the can it says that the product is not recommended for surfaces on which polyurethane will be used. For such applications they recommend Bulls Eye Sealcoat. I generally use two coats of Minwax satin polyurethane on "difficult" surfaces, both to protect them and to add a bit of smoothness and uniformity to the result, and it was my intent to do so with these "ugly" AR-3as. I did a bit of Googling to see whether I could get away with polyurethane as a last step, encouraged by the fact that I would be diluting the shellac with denatured alcohol. I found that many people had written on the subject, and the consensus was that polyurethane worked OK over stained surfaces that had been "primed" with shellac. The biggest issue seemed to be drying time, and this didn't involve polyurethane at all: Several writers lamented that stain seemed to take an unusually long time to dry when applied over shellac, so I took this into account when planning my work on the AR-3as.
The quart can of Zinsser Bulls Eye Shellack cost me something like $14 at Home Depot. The denatured alcohol cost something like $5.
As always, I sanded the speaker cabinets down until there was no (or as little as possible) old stain or other foreign substances on them. I applied the shellac/denatured alcohol mix with a clean rag, thick enough that I could clearly see where it was, but thin enough that there were no puddles or streaks. Zinsser Bulls Eye Shellac has a pretty thin, watery consistency, perhaps a bit thinner than Minwax Satin Polyurethane, and it dried quickly. I only used one coat, as I didn't want to undermine the stain's ability to bind with the speaker's surfaces, and I probably could have started staining that night, but I chose to wait a day. The surfaces dried with a light sheen.
When I started staining the next evening (I used Minwax oil-based Red Mahogany stain), I could see immediately that the shellac had been a good idea. The contrast between light and dark was reduced, relative to plywood I had stained without shellac in the past, and the grain looked fairly attractive given that it was pine plywood. I did a couple of surfaces on each speaker and then let them sit, wondering how long it would take them to dry.
As it turned out, the folks on the internet had been right. Even with a single coat of stain over diluted shellac, I could still feel some tackiness after a day-and-a-half. The weather had been unusually warm, and this probably contributed to drying time, but I called the speakers' owner and let him know this wasn't going to be a quick project.
After three or four days, I was finally comfortable turning the speakers upside down and staining the other surfaces. I let them sit for another three days, stained the fronts, and then waited a few more days before starting to apply polyurethane.
I applied at least two coats of polyurethane to each surface (more to the tops and bottoms) and the final result was a perfectly nice looking pair of Acoustic Research AR-3a loudspeakers. Do they look as nice as real walnut veneer AR-3as? I'd have to say no, but an audiophile friend who collects Acoustic Research speakers came over and was impressed with their overall appearance.
I considered using Zinnser Bulls Eye Shellack instead of my usual polyurethane over the stain on the AR-3as, but I had read that shellac scratches more easily than polyurethane and also doesn't repel water that well. In fact, I had read that water can lead to white marks that can only be removed by wiping down the surface with denatured alcohol and then applying another coat of shellac. So I stuck with my usual poly for the final coats.
This is the only time I've used shellac on the speakers I refinish, and it did what it was supposed to do. Diluted 4 to 1 with denatured alcohol, it allowed the stain to go on with a relatively high degree of uniformity, leading to plywood surfaces that probably look about as nice as they can. It did, I think, add to the drying time for the stain I used, so wood workers who plan to use shellac as I did should plan accordingly.
Clean up was easy for me, as I used an old (but clean) rag for application. Brushes can be cleaned with denatured alcohol or ammonia. If your brush is stiff next time you're ready to use it, denatured alcohol should loosen it up.
I hope others who have used shellac in other applications will write reviews as well. I have an almost-full quart can here, waiting for other jobs.