Introduction
In the world of finishing wood projects, there are often conflicting opinions on what techniques work best. One such debate revolves around whether or not it is advisable to use shellac as a sealer under a polyurethane finish. Many experts claim that using shellac under polyurethane can lead to adhesion issues due to the wax content in shellac. However, in this article, we aim to put this myth to rest by conducting an experiment and testing the effectiveness of using shellac under polyurethane.
Understanding Shellac Products
Before we dive into the experiment, it’s important to understand the various types of shellac products available in the market. The most common brand is the Bullseye Shellac by Zinsser. They offer two main varieties - amber and clear (also known as blonde). The amber shellac adds a noticeable color to the wood, whereas the clear shellac brings minimal color. Additionally, there are two types of shellac - waxed and dewaxed. The waxed shellac is ideal for building coats of finish, while the dewaxed shellac, also known as Bullseye Sealcoat, is more suitable for sealing applications.
Conducting the Experiment
To test the effectiveness of using shellac under polyurethane, the Wood Whisperer decided to create three test boards. One board was coated with dewaxed shellac, another with waxed shellac, and the third with only polyurethane. The shellac was applied as a thin layer to mimic real-world usage, where it is primarily used as a sealer coat before applying a topcoat of polyurethane.
Applying the Finishes
To ensure consistency in the experiment, the dilution of the shellac was calculated using a woodshop widget calculator. The dewaxed shellac was diluted to a two-pound cut, while the waxed shellac remained at a three-pound cut. The finishes were applied using a brush, with the dewaxed shellac being applied first, followed by the waxed shellac and polyurethane.
Results and Observations
After allowing the finishes to dry, it was clear that there were no adhesion issues with any of the test boards. Peeling off various materials like duct tape and aluminum tape resulted in no damage to the finish. However, when epoxy was applied directly to the surface, it caused the finish on the board with waxed shellac to peel up, leaving bare wood exposed. This indicates that while shellac under polyurethane is generally effective, caution should be exercised when exposing the finish to certain chemicals like epoxy.
Advantages of Using Shellac Under Polyurethane
Using shellac as a sealer under polyurethane offers several advantages. Firstly, a coat of shellac acts as a barrier between the wood and the polyurethane, preventing stain penetration and ensuring a more even finish. Shellac also dries quickly, allowing for faster project completion. Additionally, the shellac seal coat provides exceptional adhesion for the subsequent layers of polyurethane, resulting in a more durable and long-lasting finish.
Conclusion
In conclusion, the experiment conducted by the Wood Whisperer proves that using shellac as a sealer under polyurethane is a viable option for wood finishing. While there may be concerns about the wax content in shellac causing adhesion issues, this experiment debunks those myths. However, it is important to note that certain chemicals like epoxy can still cause the finish to peel up, so caution is advised. Overall, using shellac under polyurethane provides numerous benefits, such as improved adhesion, faster drying time, and a more even finish.
