In the world of woodworking, there’s an ongoing debate that’s as old as the craft itself. It’s about the use of polyurethane over shellac. This topic is not just a matter of preference but also about understanding the science behind these materials and their interaction. The question is, can you apply polyurethane over shellac? And if so, how? Let’s dive into this intriguing subject and shed some light on it.
Understanding Shellac and Polyurethane
Before we delve into the heart of the matter, it’s crucial to understand what shellac and polyurethane are, their properties, and how they’re used in woodworking.
Shellac: The Natural Finish
Shellac is a natural resin secreted by the female lac bug, found in the forests of India and Thailand. It’s been used for centuries as a finish for wood and other materials. Shellac is known for its fast drying time, easy application, and the warm, rich glow to wood. It’s also food-safe, making it a popular choice for cutting boards and children’s toys.
However, shellac isn’t without its drawbacks. It’s not as durable as some other finishes and is sensitive to heat and alcohol. That means a hot coffee or a spilled glass of wine can damage a shellac finish.
Polyurethane: The Tough Protector
On the other hand, polyurethane is a synthetic finish known for its durability. It’s resistant to water, heat, and many common household chemicals, making it a popular choice for floors, kitchen tables, and other high-use items.
Polyurethane comes in two types: oil-based and water-based. Oil-based polyurethane offers a warm, amber glow similar to shellac, but it takes longer to dry and has a stronger odor. Water-based polyurethane dries quickly and has a clear finish, but it doesn’t enhance the wood grain as much as oil-based polyurethane.
The Great Debate: Shellac vs. Polyurethane
So, why the controversy about applying polyurethane over shellac? Well, it comes down to the differences between these two finishes. Shellac is a natural, breathable finish that’s easy to repair but not as durable. Polyurethane is a tough, protective finish that’s more difficult to repair but offers superior resistance to damage.
Some woodworkers swear by the beauty of a shellac finish and believe that applying polyurethane over it is akin to putting plastic over a masterpiece. Others argue that the durability of polyurethane makes it a better choice, especially for items that see a lot of use.
The Debate: Can You Put Polyurethane Over Shellac?
The woodworking community is divided on the issue of applying polyurethane over shellac. Some woodworkers are staunch advocates of this method, while others caution against it. Let’s explore both sides of the argument.
The Case Against Polyurethane Over Shellac
Those who advise against applying polyurethane over shellac often cite compatibility issues. They argue that polyurethane, being a synthetic finish, may not adhere well to shellac, a natural finish. This could lead to peeling or chipping over time, especially in high-use items.
Moreover, shellac’s sensitivity to heat and alcohol is another concern. If a shellac-coated item is exposed to these elements, it could cause the shellac layer to become compromised, potentially affecting the polyurethane layer applied over it.
The Case For Polyurethane Over Shellac
On the other hand, proponents of applying polyurethane over shellac argue that it combines the best of both worlds. They believe the shellac layer enhances the wood’s natural beauty, while the polyurethane layer provides the necessary durability and protection.
These woodworkers often point out that shellac has been used as a sealer under other finishes for centuries. They argue that if the shellac layer is properly prepared, and the polyurethane is applied correctly, there should be no issues with adhesion or durability.
The Role of Dewaxed Shellac
This is where dewaxed shellac enters the picture. Dewaxed shellac is shellac that has had the naturally occurring wax removed. This wax can interfere with the adhesion of other finishes, so removing it can make shellac more compatible with finishes like polyurethane.
The Benefits of Using Dewaxed Shellac
Dewaxed shellac offers several benefits when used as a base for polyurethane. First, it retains all the positive qualities of regular shellac, such as its fast drying time and the warm glow it imparts to wood.
Second, because it’s free of wax, dewaxed shellac is less prone to issues with adhesion when other finishes are applied over it. This makes it a more reliable choice for those who want to apply polyurethane over shellac.
Step-by-Step Guide: Applying Polyurethane Over Shellac
Applying polyurethane over shellac can be a straightforward process if done correctly. Here’s a step-by-step guide to help you achieve a smooth, durable finish.
Step 1: Prepare the Wood
Start by sanding the wood surface to remove any rough spots. Use fine-grit sandpaper (220-grit is a good choice) and sand in the direction of the wood grain. Wipe off the dust with a damp cloth.
Step 2: Apply the Shellac
Stir the shellac well before applying. Use a high-quality brush and apply the shellac in thin, even coats. Allow each coat to dry thoroughly before applying the next. Two to three coats should be sufficient for most projects.
Step 3: Lightly Sand the Shellac
Once the final coat of shellac is dry, lightly sand the surface with 320-grit sandpaper. This will help the polyurethane adhere better. Wipe off the dust with a damp cloth.
Step 4: Apply the Polyurethane
Stir the polyurethane well before applying. Use a high-quality brush and apply the polyurethane in thin, even coats. Allow each coat to dry thoroughly before applying the next. Two to three coats should be sufficient for most projects.
Step 5: Sand Between Polyurethane Coats
After the first coat of polyurethane is dry, lightly sand the surface with 320-grit sandpaper. Wipe off the dust with a damp cloth. Repeat this process for each subsequent coat of polyurethane.
Step 6: Let it Cure
Once the final coat of polyurethane is dry, let the piece cure for several days before using it. This will allow the finish to harden and provide maximum protection.
Common Mistakes to Avoid When Applying Polyurethane Over Shellac
When applying polyurethane over shellac, there are a few common mistakes to avoid.
Not Preparing the Wood Properly
Proper preparation is key to a successful finish. Be sure to sand the wood thoroughly and remove all dust before applying the shellac.
Applying Thick Coats
Both shellac and polyurethane should be applied in thin, even coats. Applying thick coats can lead to drips, runs, and an uneven finish.
Not Allowing Coats to Dry
Patience is a virtue in woodworking. Each coat of shellac and polyurethane needs to dry thoroughly before the next is applied. Rushing this process can lead to a sticky, uneven finish.
Using Waxed Shellac
As discussed earlier, dewaxed shellac is the best choice when applying polyurethane over shellac. Using waxed shellac can lead to adhesion issues.
Shellac and Polyurethane: A Comparison of Durability and Protection
Shellac and polyurethane, used as finishes in woodworking, offer different levels of durability and protection.
Shellac, being a natural finish, provides a beautiful, rich glow to the wood but falls short in durability. It’s sensitive to heat and alcohol, which can cause it to cloud, soften, or even dissolve. Its moderate protective qualities suit items not exposed to heavy wear or harsh conditions.
Polyurethane, on the other hand, is a powerhouse regarding durability and protection. It’s resistant to water, heat, and many common household chemicals. It is an excellent choice for high-traffic items like floors or kitchen tables. However, it doesn’t enhance the wood’s grain and color as much as shellac.
Applying polyurethane over shellac combines the best of both worlds. The shellac layer enhances the wood’s natural beauty, while the polyurethane layer provides the necessary durability and protection. This combination results in a visually appealing and highly durable finish.
Exploring Other Finishes Over Shellac
While polyurethane is a popular choice, it’s not the only finish that can be applied over shellac. Other options include lacquer and varnish.
Lacquer, like shellac, is a natural finish that dries quickly and imparts a beautiful sheen to the wood. It’s more durable than shellac but less so than polyurethane. Lacquer can be applied over shellac, but it’s essential to use dewaxed shellac to avoid adhesion issues.
Varnish is another durable finish that can be used over shellac. It’s slower drying than shellac and lacquer but offers excellent protection against water and UV rays, making it a good choice for outdoor furniture.
Compared to polyurethane, both lacquer and varnish can provide similar levels of protection, but they each have unique characteristics. Lacquer offers a high-gloss finish and is easier to repair than polyurethane, while varnish provides superior UV protection, making it better suited for outdoor use.
Product Recommendations: Best Polyurethanes to Apply Over Shellac
When applying polyurethane over shellac, not all products are created equal. Here are a few recommendations for high-quality polyurethanes that work well over shellac.
Minwax Fast-Drying Polyurethane
Minwax is a trusted name in wood finishes, and its Fast-Drying Polyurethane is no exception. It offers long-lasting durability and protection and is available in clear gloss and satin finishes. This product can be found at most home improvement stores and online.
Varathane Triple Thick Polyurethane
This product from Varathane is three times thicker than traditional polyurethane, allowing you to achieve the same level of protection with fewer coats. It’s also water-based, making cleanup a breeze. Varathane Triple Thick Polyurethane can be purchased online at most home improvement stores.
General Finishes High-Performance Water-Based Topcoat
This topcoat from General Finishes is another excellent choice for applying over shellac. It’s water-based, dries quickly, and provides a durable, protective finish. It’s also available in various sheens, from flat to gloss. This product can be found at specialty woodworking stores and online.
Expert Opinions: What Professionals Say About Applying Polyurethane Over Shellac
Professional woodworkers have varied opinions on applying polyurethane over shellac, but there’s a consensus that it can work well if done correctly.
Bob Flexner, a renowned finishing expert and author of “Understanding Wood Finishing,” advocates for using dewaxed shellac under other finishes, including polyurethane. He emphasizes that the key is to ensure the shellac is dewaxed to avoid potential adhesion issues.
Similarly, Michael Dresdner, a professional woodworker and author of “The New Wood Finishing Book,” supports using shellac as a sealer under other finishes. He points out that shellac’s ability to adhere to almost anything and allow almost anything to adhere to it makes it an excellent sealer.
These expert opinions underscore the importance of understanding the materials you’re working with and taking the time to prepare and apply them correctly.
FAQs
Can I use any shellac under polyurethane?
It’s best to use dewaxed shellac under polyurethane. The wax in regular shellac can interfere with the adhesion of the polyurethane.
Can I use water-based polyurethane over shellac?
Yes, you can use water-based polyurethane over shellac. However, ensure the shellac is fully cured before applying the polyurethane to avoid potential adhesion issues.
How long should I wait between applying shellac and polyurethane?
Allow the shellac to fully cure before applying polyurethane. This typically takes a few hours but can vary based on the product and environmental conditions.
Can I apply polyurethane over shellac on outdoor furniture?
While you can apply polyurethane over shellac on outdoor furniture, remember that shellac is not as durable or water-resistant as other finishes. It’s best to use a spar urethane, designed for outdoor use, over the shellac for maximum durability.
Troubleshooting: Solving Common Problems When Applying Polyurethane Over Shellac
Applying polyurethane over shellac can sometimes present challenges. Here are some common problems and how to solve them.
Polyurethane is Peeling or Chipping
If your polyurethane is peeling or chipping, it could be due to poor adhesion. Ensure you’re using dewaxed shellac and allowing it to fully cure before applying the polyurethane. Also, lightly sand the shellac before applying the polyurethane to improve adhesion.
The finish is Streaky or Uneven
Streaky or uneven finishes can be caused by applying too thick a coat or not allowing each coat to dry fully before applying the next. Remember to apply thin, even coats and be patient in allowing each coat to dry.
Bubbles in the Finish
Bubbles can form if you shake the polyurethane before applying it or if you apply it with a foam brush. Stir the polyurethane gently before use, and consider using a high-quality bristle brush for application.
Final Verdict
Applying polyurethane over shellac is a topic that has sparked much debate among woodworkers. However, the consensus is that it can be a successful method if done correctly. The key is understanding the properties of both shellac and polyurethane and taking the time to prepare and apply each finish properly.
Shellac brings out the natural beauty of the wood, while polyurethane provides a durable, protective layer. When combined, they can create a finish that is both visually appealing and long-lasting.
