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When it comes to finishing woods, the options can be overwhelming. If you’re trying to decide whether to use shellac or polyurethane on your next project, you’ve come to the right place.
In this article, we’ll introduce you to these two finishes and compare their qualities. We’ll review both similarities and differences, and define the major differentiating factor between shellac and polyurethane. Finally, we’ll teach you when to use shellac, when to use polyurethane, and discuss which is better.
- What Is Shellac?
- What is Polyurethane?
- Shellac vs Polyurethane
- Major Differentiating Factor
- When to Use Shellac
- When to Use Polyurethane
- Which Is Better, Shellac or Polyurethane?
What Is Shellac?
Shellac is a type of lacquer. It is a non-curing finish, which means it hardens solely through environmental means.
Shellac contains just two ingredients; resin and denatured alcohol. The denatured alcohol acts as a solvent to make the resin spreadable, then evaporates, leaving behind a lightly protective film finish.
Shellac has a long history of use as a finish for valued decorative objects. While it is not resistant to heat and can only tolerate a limited amount of water for a short period of time, shellac does provide protection from dings and scratches.
Because shellac is made from a naturally occurring resin, harvested from trees inhabited by the lac beetle, it does not emit VOCs, making it one of the safest finishes available. It is also non-toxic and food safe, and can even be used as a food glaze.
Shellac imparts a warm glow and gentle sheen wherever it is applied. It highlights the natural features of the wood, such as the color and grain pattern.
Each layer of shellac dissolves into the one beneath it, which eliminates the need for sanding between layers and considerably speeds the application process.
Within seconds of brushing or padding shellac on, the denatured alcohol evaporates, leaving behind a layer of dry film finish. When another layer is added, any imperfections in the lower coat dissolve, making this one of the easiest and most fool-proof finishes to apply.
When untinted, the color of shellac can range from a pale yellow to a deep garnet. Tints can be added, and shellac is noted for its ability to hold and preserve tint.
What is Polyurethane?
Polyurethane is a type of varnish. Varnishes are curing finishes, so they harden through both environmental and chemical methods. The evaporation of solvents is responsible for the initial hardening of polyurethane.
In a matter of days to weeks, depending on the formulation of the polyurethane, a process called crosslinking occurs that bonds individual molecules more tightly together. When this process has completed, the polyurethane is cured. Once fully cured, polyurethane is food-safe.
Polyurethane is the most durable of all the available finishes (aside from maybe spar varnish), and is often used as a finish for wood floors. Even heavy foot traffic or shifting furniture shouldn’t disturb this super-strong layer, as long as you wait until it’s fully cured to walk on it.
Two formulations of polyurethane are available for purchase. One has an oil base, while the other contains water.
- Water-based polyurethane dries clear in just a few hours, and cures within a few days. Water polyurethane is slightly less durable and long lasting than oil-based versions.
- Oil-based polyurethane also dries clear, but takes up to 24 hours before another coat can be applied. It takes several weeks to fully cure.
Both types of polyurethane are waterproof and protects wood from fungus and mildew in addition to deflecting abrasive damage.
Oil-based polyurethane emits high levels of VOCs. Water versions are usually much lower or VOC free, but contain isocyanates. A respirator is mandatory when working with either formulation of polyurethane.
They can be removed by using a ‘tip off’ technique that uses the bristles of the brush to pop the bubbles. Thinning polyurethane before applying will encourage any bubbles to pop on their own. Should the bubbles (or any other imperfections) harden, they need to be sanded off before another layer is added.
When three or four thin coats of poly are applied using proper technique, the result is a super-hard, clear film finish. By default, polyurethane is a high-gloss finish. Deglossers can be added to create products with less shiny or even matte finishes.
Shellac vs Polyurethane
Learn what shellac and polyurethane have in common, and also some of the key differences between these two finishes.
Both shellac and polyurethane are thinnable, film-forming finishes that are best used in interior application.
Both shellac and polyurethane are film finishes, which means that they form a layer of protection that sits on top of the wood. This is in contrast to oil finishes that are rubbed into the wood and do not form a film layer.
Polyurethane and shellac are both best restricted to use indoors. Shellac will cloud upon extended contact with water. (Less than about four hours is okay, allowing shellac to be used on salad bowls and cutting boards.)
While polyurethane, a waterproof finish, might seem ideal for use outdoors, it’s actually too stiff and inflexible to handle the expansion and contraction that exterior wood experiences as the temperature and relative humidity change.
Both shellac and polyurethane can be thinned with solvents to produce a less viscous liquid finish. A thinned solution is often used for the first coat, called the seal coat.
Polyurethane looks best when applied in several thinner layers. The ratio of thinner to finish also depends on the weather at the time of application -more solvent may be needed to slow dry time on hot days.
While polyurethane and shellac have a few important things in common, there are many differences as well.
Shellac has a subtle, natural shine that enhances the look of decorative wood pieces. Polyurethane has a more plastic-like appearance, especially when applied in thick layers.
Ease of Repair
Polyurethane is both durable and long-lasting, meaning repairs should be unnecessary. When it is time to replace polyurethane, the finish must be completely removed and reapplied.
Shellac doesn’t last as long as polyurethane, but it can easily be revived, refreshed, or repaired with a another thin coat of shellac. Shellac is also relatively simple to remove if needed.
Once shellac is mixed, it has a very short shelf life. For this reason, shellac is usually only purchased as needed, or is formulated in the workshop in the amount needed for the job at hand.
Polyurethane is shelf-stable and can last for years when unopened or properly stored.
Ease of Application
To achieve a smooth finish, polyurethane must be sanded between coats, which also serves to provide a grippable surface for the next layer of polyurethane.
Shellac is a self-correcting finish, so imperfections can easily be eliminated simply by adding another layer of shellac.
Polyurethane also has a tendency to bubble, which shellac does not.
Mechanism of Action
As explained above, polyurethane is a curing finish. It hardens in two phases, creating an extra-strong and durable protective layer over wood. Shellac is non-curing. Once it dries, it is as hard as it will ever get.
Shellac dries extremely quickly, less than a minute after it is applied. Water polyurethanes take several hours to dry. Oil-based polyurethanes have the longest dry time, and you may need to wait 24 hours between coats.
Shellac is essentially VOC-free. It is the least toxic finish available. Oil-based polyurethane is among the most toxic varnishes, emitting high levels of VOCs during application and for a significant time afterward. Water polyurethane has low to no VOCs.
Resistance to Heat
Polyurethane is a reasonable choice for dining tables or eating surfaces, as it is not affected by liquid or low levels of heat, such as a hot mug or serving dish. Shellac, however, shows waxy white rings wherever a hot beverage has been set down.
Major Differentiating Factor
When you need to choose between shellac and polyurethane, ask yourself how much wear and tear the wood is likely to see. Polyurethane provides superior protection from impact damage, abrasion, water, fungus, mildew and rot.
Shellac protects wood from pests, can resist some water, and seals in odor, but it does not last as long or repel as much damage as polyurethane, making it less suitable for work surfaces and flooring.
When to Use Shellac
Use shellac to provide light protection and enhance the look of wood.
Antique furniture from the Victorian era (1800s through about 1920 or 1930) was almost certainly finished with shellac, as this was the most popular finish at the time. Applying shellac will preserve the original character of the piece.
For new furniture that won’t see heavy use, such as a side table, shellac is the perfect way to highlight the natural beauty of the wood.
When to Use Polyurethane
Use polyurethane to provide super-durable protection for any surface.
Bar tops or wooden countertops are often coated in polyurethane thanks to it’s spill-proof, wipeable surface.
Because polyurethane is so durable, it is the finish of choice for floors and table tops. It is able to stand up to considerable wear and tear while still looking great, and it lasts a long time.
Which Is Better, Shellac or Polyurethane?
Polyurethane is better when a waterproof, heatproof, highly durable protective barrier is your goal.
To enhance wood, provide some protection, and achieve a natural, subtle finish, shellac is the better choice.