Polyurethane 101 - How to Apply Polyurethane
Polyurethane is a durable and easy to work with protective wood finish. It is a polymer that is actually a form of plastic that seals the wood and protects it. This guide will walk you through the process of applying polyurethane properly to achieve a beautiful, long-lasting finish on your woodworking projects.
Understanding Polyurethane Finishes
Polyurethane can be found in both oil-based and water-based formulas. The majority of my projects use oil-based polyurethane because I prefer the slightly warmer tone that it provides.
Oil-based has a shelf life of about 3 years and I always have a can available. You can buy polyurethane in a number of sizes but I tend to keep a quart handy.
There are also spray-application polyurethanes, but I tend to avoid them. Polyurethane is most commonly applied with a brush but a rag can work as well. A minimum of two (2) coats should be applied but I recommend at least three (3) coats for most projects.
Choosing your base (water based vs oil based):
As mentioned above, I tend to use oil-based polyurethane, it pulls the natural beauty from the wood and offers a deep warm (but not tacky) glow. Oil-based polyurethane typically requires fewer coats and is a bit thicker. The downside is that oil-based polyurethane will take longer to dry (8 hours between coats for Oil vs. 2 hours between coats for water) – this leaves your project susceptible to dirt, debris and dust. It also has the tendency to show brush marks more easily. Both of the negatives can be avoided by a little planning and proper application. I have been burned by brush marks on my application a few times but these shortfalls are not permanent – if this happens then just wait for the polyurethane to dry and sand it off (while wearing your mask!).
Safety Precautions
Wear your gloves and mask, try to vent the room as much as possible.
Yeah, I get it gloves are a nuisance, you have to remember to have them around and it seems so much easier to just dive into your project, after all – you have done this before and you will be careful, right?
The truth is that the amount of time that you will spend cleaning polyurethane off of your skin will greatly exceed the time you spend finding and putting on your gloves. Just do it, take the time and put them on.
Polyurethane, a petrochemical resin that contains isocyanates, is a known respiratory toxin, i.e., smart people think you should wear a mask, so do it. You should have air circulating as best as you can. My prior workshop was a basement and ventilation was at a premium. If my projects required a heavy dose of Polyurethane then I would wait until it was a nice day and take this step outside. For garage workshops just open up the garage and turn a small fan on to keep the air moving. Don't point the fan directly at the project, this may lead to uneven drying and you risk blowing debris on the project that will be a nuisance to remove.
Step-by-Step Polyurethane Application Guide
Follow these steps for a perfect finish every time
Understand Polyurethane
Polyurethane is a durable protective wood finish. It's a polymer (plastic) that seals and protects wood. Available in oil-based and water-based formulas, with oil-based providing a warmer tone but longer dry times.
Safety First
Always wear gloves and a mask. Polyurethane contains isocyanates which are respiratory toxins. Ensure good ventilation in your workspace, using fans to circulate air but not blowing directly on your project.
Sand Properly
Start with rougher grit (80) and progress to finer (120, then optionally 220). Always sand with the grain and clean thoroughly with a damp lint-free rag to remove all dust before applying polyurethane.
Don't Shake the Can
One of the biggest enemies of the application process is bubbles. Do not shake the can before use, as this introduces air bubbles. If the can was jostled during transport, let it settle before opening.
Apply Thin Coats
Less is more! Apply multiple thin coats rather than one thick coat. Heavy application leads to dripping and uneven drying. Oil-based polyurethane goes on milky but dries clear.
Avoid Overbrushing
Use long straight strokes along the grain and avoid overbrushing, which can cause brush marks. Trust that it will dry smoothly rather than trying to fix areas that look uneven while wet.
Sand Between Coats
After the first coat dries (check label for times), lightly sand with 220-grit sandpaper. Don't push hard, just a light sanding along the grain to create a tooth for the next coat to adhere to.
Repeat Process
Clean off sanding dust and apply additional coats (2-3 total). Remember that drying times (1 day) differ from cure times (20-30 days). You can use the finished surface during curing, but be cautious until fully cured.
Common Problems and Solutions
Bubbles in the Finish
Caused by: Shaking the can, applying too quickly, or using a foam brush
Solution: Don't shake the can (stir gently instead), apply slowly and methodically, use a high-quality natural bristle brush for oil-based or synthetic brush for water-based
Brush Marks
Caused by: Overbrushing, using too thick of a coat, or using low-quality brushes
Solution: Apply with long, smooth strokes along the grain, use thin coats, invest in a good brush, and consider adding a product like Penetrol to oil-based polyurethane to improve flow
Dust Nibs
Caused by: Dust settling on wet finish
Solution: Ensure your working area is clean, consider working in a dust-free environment, and lightly sand between coats to remove any dust nibs
Uneven Sheen
Caused by: Inconsistent application or improper stirring of the polyurethane
Solution: Stir (don't shake) thoroughly before and occasionally during application, apply consistent thin coats
Conclusion
Applying polyurethane can seem intimidating at first, but with these guidelines and a bit of practice, you'll be able to achieve professional-looking results. Remember that patience is key—rushing through the process often leads to mistakes that require more time to fix. Take your time, apply thin coats, and allow proper drying time between coats for a beautiful, durable finish that will protect your woodworking projects for years to come.
Remember that drying times and cure times are completely different. Your project will dry within a day but it will not fully cure for 20-30 days. This doesn't mean that you can't use the finished surface but it does mean that you should be cautious for the first month.