Polyurethane is a popular choice for finishing woodworking projects due to its durability and beautiful finish. However, it is not uncommon to encounter issues such as bubbles, puddles, runs, and brush marks when applying polyurethane. In this article, we will guide you through the process of fixing these flaws, ensuring a flawless finish that meets your expectations.
As an experienced woodworker with over 30 years of experience, Paul understands the frustrations that can arise when flaws appear in a polyurethane finish. He acknowledges that manufacturers often oversimplify instructions or have limited space on product labels to provide detailed guidance. Therefore, Paul has conducted extensive experimentation and research to uncover effective methods for correcting various types of flaws.
Before delving into the solutions, it is important to note that a good finish requires a solid foundation, which begins with proper sanding. Most woodworkers dread the sanding process, but it is crucial for achieving a smooth and flawless finish. Paul strongly recommends watching his videos on sanding techniques, such as “Sanding Efficiently” and “Choosing the Right Grade Sandpaper for Your Woodworking Project,” as they provide valuable insights on making the most out of your sandpaper.
Now, let’s move on to the key points for fixing specific flaws:
1. Puddles and Runs
Puddles and runs occur when there is an accumulation of polyurethane, resulting in a thick area that stands out from the rest of the surface. To address this issue, Paul suggests wiping the affected area with a rag soaked in mineral spirits or paint thinner. For oil-based polyurethane, you have approximately 20 minutes before it starts to dry. In contrast, water-based polyurethane should be wiped within 5-10 minutes to prevent drying. By acting quickly, you can remove the excess polyurethane before it fully hardens, ensuring a smooth and even finish.
2. Bubbles
Bubbles in polyurethane can be classified into two categories: small bubbles, which are smaller than the head of a pin, and large bubbles, which are larger than the head of a pin. The recommended method of correction varies depending on the size of the bubbles.
For small bubbles, Paul suggests using a random orbital sander with 120-grit sandpaper to gently sand the surface. It is crucial to remove the high spots before the polyurethane fully hardens, as it becomes more challenging to remove afterward. Be prepared for the sandpaper to become gummed up during the process, requiring occasional changing.
For larger bubbles, Paul recommends using a methyl chloride-based stripper. It is essential to exercise caution when using this powerful stripper, as it emits fumes that can be harmful. Apply the stripper generously and let it sit for 15 minutes. Then, gently scrape away the stripped polyurethane and wipe the surface with lacquer thinner to remove any residue.
3. Brush Marks
Brush marks can appear when laying down a coat of polyurethane with a brush. To minimize brush marks, Paul advises choosing a slower-drying polyurethane, as it allows the brush marks to level out naturally. Fast-drying polyurethane tends to trap brush marks, bubbles, and puddles, leading to an uneven finish. Take your time during the application process, ensuring smooth and even brush strokes to minimize the appearance of brush marks.
By following these methods, you can address common flaws in polyurethane finishes and achieve a flawless outcome. Remember to also watch Paul’s video on “Bubble-Free Polyurethane Application Technique” for additional tips and techniques for achieving a bubble-free finish.
In conclusion, Paul’s expertise and dedication to finding solutions to common polyurethane flaws have resulted in an informative guide for woodworkers. By understanding the proper techniques for addressing bubbles, puddles, runs, and brush marks, you can achieve a flawless polyurethane finish that meets your expectations. Remember to prioritize proper sanding techniques as the foundation for a perfect finish. With the knowledge gained from this article, you can confidently tackle any polyurethane finishing project.
