Polyurethane is an extremely popular wood furniture finish known for its durability and many other advantages. For the glossiest finish, some people recommend sanding between coats. Is this step necessary, and why?
You should sand between coats of polyurethane, even if it is not absolutely necessary for some types. Sanding between coats will create a better mechanical bond between coats and protect the wood better. Additionally, sanding will give you the glossiest finish possible.
In this article, I will cover the standard recommendations for different types of polyurethane wood finishes, the reason sanding between coats is highly recommended, and pro tips to help you master the technique of sanding.
Which Types of Polyurethane Require Sanding Between Coats?
Not all polyurethane wood finish is created equally. You’ll find that there are several differences between the various types available. For example, applying polyurethane on epoxy will leave a yellow hue. When it comes to sanding between coats, it is an absolute requirement for some types of polyurethane finish and optional for others.
Oil-based polyurethane requires sanding between coats. As for water-based polyurethanes, every manufacturer has different recommendations. I prefer to stay on the safe side and sand anyway because it is a good practice and will ensure you get the best results.
Why Sanding Between Coats Is Important
Sanding takes up a lot of time and patience. Before you put in all that effort to sand between applications of polyurethane, you should know why it is important. Here’s what happens when you follow this practice:
When the polyurethane is sanded down, it is smooth enough for a stronger mechanical bond to form between subsequent coats of polyurethane. Better bonding means increasing the lifespan and durability of your furniture piece. In other words, you won’t need to reapply the finish anytime soon.
One of the major reasons that wood finishes are used instead of leaving wooden furniture bare is the visual aspect. Polyurethane finish leaves a comparatively dull sheen on the wood, in contrast to wood stains and other finishes. However, sanding between coats of polyurethane greatly improves the glossy appearance of finished pieces.
Problems That Occur When You Don’t Sand Between Coats
Besides the advantages of sanding between coats, it’s important to realize what would happen to your woodwork if you skipped sanding altogether.
Since polyurethane is applied using a brush, even professional woodworkers are bound to leave brush marks behind. Sanding will remove all brush marks and leave you with a smooth surface to apply the next coat of finish. If you skip sanding, there is no way to remove the brush marks, and they will probably be visible in the final product.
The Next Coat Won’t Adhere Well
As I’ve mentioned above, sanding allows the polyurethane layers to create a better mechanical bond which increases durability. On the other hand, if you skip sanding, the subsequent layer of polyurethane won’t form very strong bonds with the layer underneath. This results in less adhesion between the layers, which might lead to peeling, and you can expect the finish to get damaged easily.
Dust and Bubbles
Polyurethane finishes need a lot of time to cure, which is enough time for the dust to settle on the surface. If you apply a coat of polyurethane on a dusty surface, it will show in the final result and also lead to bad adhesion.
Dust between layers can also trap air bubbles in the polyurethane, which will take you a step away from the perfect glossy finish you’re after. Once the polyurethane sets, removing dust and bubbles trapped between the layers is impossible. The only thing you could do is strip the wood finish and start over.
Tips on How To Sand Between Coats of Polyurethane Finish
Now that we’ve established the importance of sanding between applications of Polyurethane, you may want to work on your sanding technique. Following the tips below will help you get the best results.
As a versatile, starter tool, recommend using the BLACK+DECKER Random Orbit Sander (available on Amazon.com), since it’s lightweight, so it’s not difficult to control. It also comes with an excellent dust collection system to keep your workspace clean.
Make Sure the Polyurethane Is Fully Dry
Just as you would do when sanding down paint between coats, it’s crucial that the polyurethane coat you’ve already applied is bone dry. Check the manufacturer’s instructions for specific timelines. Generally, water-based polyurethanes dry in 2 hours, while oil-based polyurethanes take longer—3 to 4 hours.
I highly suggest that you wait an extra half hour (after the recommended time according to the manufacturer). This will allow you to be confident that the polyurethane has dried and it’s ready for sanding.
Work From One End to the Other
Taking a systematic approach to sanding will ensure that you don’t skip any parts. If you skip one area while sanding, it will be highly visible after the second and third coats are applied. To ensure that you are covering the entire surface area of the wood, experts recommend moving from one corner to the next.
Follow a zig-zag pattern for the best results. This ensures that you do not sand the same place twice and that you cover the entire area without missing any spots.
Using very coarse sandpaper will shave off too much material from the wood. You do not need to be polishing the polyurethane between coats, but you also don’t need to remove the entire layer. So, pay close attention to the grit size of the sandpaper you will be using.
Medium-grit sandpaper is the best option. I recommend using 220-grit sandpaper because it will ensure that you are removing only the ridges and imperfections, leaving behind a smooth and shiny layer of polyurethane ready to bond with the next layer.
You can follow up with 320-grit sandpaper (finer) for a smoother, polished finish. I recommend only using the finer sander before the final coat because the polished finish will be more visible through the last coat of polyurethane finish.
Slow and Steady
This rule applies to sanding in general. You cannot, under any circumstances, speed up the process of sanding. The optimal method of sanding requires applying light, even pressure, on the surface, allowing it to glide on the wood instead of pressing hard into it.
You should take your time while sanding because it allows you to pay attention to detail. Additionally, slower sanding gives you more control over the sander. Otherwise, you run the risk of the sander slipping from your grip, which is dangerous, to say the least.
Use Your Hands for Tough Spots
While an electric sander is handy for larger surface areas because it gets the job done faster, it’s often too bulky to reach some areas. I recommend that you take the sandpaper off the sander and apply some elbow grease.
Areas like the inside of drawers can be difficult to sand using a bulky orbital sander. If you’re not confident using sandpaper manually, consider buying a smaller sander or a detail sander. That’s the type with a head that installs triangular-shaped sandpaper—it allows you to get into the nooks and crannies that a circular disc of sandpaper wouldn’t.
I use the BLACK+DECKER MOUSE (available on Amazon.com) for just that—detail works like the corners and other tough-to-reach places. It’s a powerful tool, and it’s compact enough for most detail jobs. It also comes with an excellent dust collection system, which is an added bonus.
Alternative Options: Polyurethane Without Sanding
I get it; sanding is difficult for many people. You may not be confident using a sander or do not own one to start with. That doesn’t mean that polyurethane is not an option for you to explore.
You can still apply water-based polyurethanes without the absolute need to sand between coats. The result will be comparatively duller, but it won’t matter.
Here are a few alternative polyurethanes to use if you don’t want to (or can’t) sand between the coats:
- Water-based polyurethanes: As mentioned earlier, sanding is optional if the finish is water-based.
- Satin finish polyurethanes: Glossy finish is very visually appealing, but it can also highlight all the flaws during the application process. Satin finish or semi-gloss finish is a type of wood finish with a less shiny final result, and sanding won’t be necessary to remove any flaws.
The Minwax One-Coat Polyurethane on Amazon.com is perfect as an alternative to sanding because you only need to apply a single coat of wood finish. It dries in two hours, making the application process very easy.
Sanding between applications of polyurethane wood finish is a recommended practice that will help you achieve the best results. For oil-based polyurethane, sanding is not optional. To get the glossiest finish that showcases the wood beneath, make sure you take the time to patiently sand after curing each coat of polyurethane.
If you can’t sand between the layers, consider using a different type of polyurethane.