In this article, we will address the question of whether or not shellac can be used as a sealer under polyurethane. Many people in the world of finishing have heard that waxed shellac should not be used under polyurethane, but is there any truth to this claim? We will conduct a few tests to determine if there are any adhesion issues when using waxed shellac as a base for polyurethane.
Before we dive into the tests, let’s first understand the different types of shellac available on the market. The most commonly found brand is “Bullseye” shellac, which comes in two varieties: amber and clear (blonde). The amber shellac brings color to the wood when applied, while the clear shellac does not add much color. Additionally, there are two different versions of shellac offered by Bullseye: waxed and de-waxed. The waxed version is a three-pound cut, which means it is relatively thick and ideal for building coats of finish. On the other hand, the de-waxed version, known as Bullseye Sealcoat, is only sold in clear and is sold in a two-pound cut. The lower cut makes it more suitable for sealing applications, as the initial coat needs to be thin and able to quickly soak in, allowing for subsequent sanding and application of top coats.
In the world of woodworking, shellac is often used as a sealer to control blotching on certain types of wood, such as cherry and pine. When these woods absorb finish unevenly, a sealer coat helps prevent dark and spotty areas. Applying shellac as a sealer before applying top coats, such as polyurethane, can improve the overall appearance of the finish. For those looking for even better blotch control, there are products like Charles Neal’s pre-color conditioner, which is a water-based product specifically designed to prevent blotching.
Now, let’s dive into the tests. We will use three test boards: one with de-waxed shellac, another with waxed shellac, and the third with just polyurethane. It’s important to note that this is not a comprehensive test, as we are only using three boards. The aim is to observe if there are any adhesion issues when using waxed shellac under polyurethane.
To ensure that our experiment is fair, we need to dilute the shellac to the appropriate cuts. Diluting shellac can be challenging due to the mixing of liquid volume and solid weight. Fortunately, there are tools available, such as the Woodshop Widget calculator, to help with dilution calculations. We dilute a small amount of shellac using the calculator and mix it well before application. It is important to use separate brushes for each shellac mixture to avoid interfering with the experiment.
After applying the first coat of shellac and allowing it to dry, we applied a second coat of polyurethane to each board. The shellac boards received their first coat of polyurethane, while the polyurethane-only board received its second coat. The advantage of using shellac as an undercoat is that it dries quickly, allowing for faster build-up of finish.
After allowing the finish to cure for a few days, we conducted some tests to check for adhesion issues. We attached duct tape, aluminum tape, and epoxy to the surface of each board, then attempted to remove these substances to observe any damage to the finish.
The results of our tests were quite promising. The polyurethane board showed no damage when the tapes were removed, indicating good adhesion. The board with de-waxed shellac also showed no damage when the tapes were removed. However, the board with epoxy applied on top of waxed shellac showed some peeling of the finish, leaving bare wood exposed.
In conclusion, our tests indicate that shellac, both de-waxed and waxed, can be used as a sealer under polyurethane without significant adhesion issues. However, it is important to note that applying a significant coat of shellac and then piling on polyurethane may lead to problems. For best results, it is recommended to use shellac as a sealer coat, applying a light layer to the raw wood before applying polyurethane as the primary topcoat.
Using shellac as a sealer can provide advantages such as quicker drying time and a more intense initial coat of polyurethane. This reduces the overall time required to build up the finish and achieve a desired level of gloss. Keep in mind that allowing the finish to cure for a few days is essential to ensure proper adhesion and durability.
In summary, if you are considering using shellac as a sealer under polyurethane, rest assured that it can be a viable option. However, be mindful of the application method and avoid excessive layering, as this may lead to adhesion issues. By following these guidelines, you can achieve a beautiful and durable finish for your woodworking projects.
