As a woodworker, you may often find yourself faced with the decision of which finishes to use on your projects. While most projects are typically coated with a single type of finish, there are times when you may wish to switch between different finishes for various reasons. However, it is crucial to understand the compatibility between different types of finishes to avoid any potential problems or failures in your woodworking projects.
In this article, we will focus on four common types of finishes that are often used in small woodworking shops: natural penetrating oils (such as boiled linseed oil and tung oil), oil-based film finishes (including polyurethane and various varnishes and lacquers), water-based polyurethane, alcohol-based shellac, and paste wax. Understanding how these finishes interact with each other will help you make informed decisions when switching finishes between coats.
1. Natural Penetrating Oils
Natural penetrating oils, such as boiled linseed oil and tung oil, are meant to soak into the wood fibers rather than create a protective shell on top of the wood. These oils take a considerable amount of time to fully cure, often weeks or even months. It is essential to distinguish between drying and curing in this context - the finish may feel dry to the touch, but curing is a chemical process that hardens the finish on a molecular level. Once the natural penetrating oil finish has fully cured, you can apply certain types of finish on top, specifically oil-based polyurethane. However, it is important to note that you cannot mix water-based finishes with natural oils like boiled linseed oil or tung oil.
To overcome the incompatibility between water-based topcoats and natural oils, you can use a barrier coat of dewax shellac. Dewax shellac, an alcohol-based finish with its natural waxy components removed, acts as a protective layer between the oil and water-based finishes. By applying dewax shellac before applying a water-based topcoat, you create a barrier that allows the two finishes to work together without any potential conflicts.
2. Oil-Based Film Finishes
Oil-based film finishes, such as polyurethane, varnishes, and lacquers, are used to create a hard protective shell on top of the wood surface. These finishes can be applied directly over natural penetrating oils like boiled linseed oil or tung oil since oil is compatible with oil. Similarly, you can apply oil-based film finishes on top of each other without any issues.
When it comes to mixing oil-based film finishes with water-based finishes, the process becomes more complex. Oil-based finishes contain thinners that evaporate during the drying process. Once fully cured, these finishes do not contain a significant amount of oil and can be overlaid with water-based finishes. Similarly, once water-based finishes have fully cured and the water has evaporated, they can be coated with oil-based finishes. However, in both cases, you may need to scuff the surface with high grit sandpaper or steel wool to create a mechanical bond between the two finishes.
3. Alcohol-Based Shellac
Alcohol-based shellac can be considered a versatile finish as it can be applied over almost any other type of finish and vice versa. Shellac is often used as a sanding sealer and can act as a barrier coat between incompatible finishes. For example, if you want to apply a water-based topcoat over an oil-based stain, an application of dewax shellac will seal the stain and prevent any potential conflicts between the two finishes.
4. Paste Wax
Paste wax, a blend of waxes softened with an oil solvent, can be applied over various finishes, including natural penetrating oils, shellac, and film finishes. However, nothing can be applied over wax itself, so if you plan to apply a different type of finish over wax, you must fully remove the wax first. Dissolving the wax with mineral spirits or another suitable solvent is necessary to ensure proper bonding of the new finish.
Understanding the compatibility and bonding characteristics of different finishes is crucial for successful woodworking projects. It is important to note that while some finishes may technically be compatible, testing the compatibility in an inconspicuous area is recommended before applying it to the entire project. Additionally, always ensure that the previous finish has fully cured before applying a new finish on top.
By following these guidelines and understanding which finishes can be mixed and which cannot, you can achieve high-quality and long-lasting finishes on your woodworking projects. Remember to always scuff the surface and create a mechanical bond when necessary, and consider the use of barrier coats such as dewax shellac to ensure compatibility between finishes.
In conclusion, by familiarizing yourself with the compatibility of different finishes and following the appropriate techniques, you can confidently switch between finishes and achieve professional results in your woodworking projects. Understanding the how and why of switching finishes between coats is essential for the success and durability of your finished pieces. With the right knowledge and techniques, you can elevate your woodworking skills and create beautiful, long-lasting projects.
Content Source: Stumpy Nubs - “The problem with mixing oil/water varnish and stain”
Keywords: mixing oil and water finishes, compatibility between finishes, woodworking finishes, natural penetrating oils, oil-based film finishes, water-based polyurethane, alcohol-based shellac, paste wax, switching finishes between coats, bonding characteristics of finishes
