Ask any woodworker, and they’ll tell you that polyurethane is one of the most useful products out there. It can be used as a protective finish or even as a very strong adhesive for your project.
There are a lot of reasons why you might want to use polyurethane, but more often than not, people want to know whether or not they have to sand the last coat.
Do you sand the final coat of polyurethane? In most cases, you don’t have to sand the last coat of polyurethane to get a smooth finish. However, if your final finish is a little bit textured, you can lightly sand the finish to smooth out the texture as well.
In this article, we’ll give you tips for getting the best results for finishing with polyurethane and answer some questions about this product.
Can You Sand Polyurethane?
The answer is yes, but first, you need to understand what polyurethane is.
Polyurethane is a class of polymers composed of organic units linked together by carbamate. It’s used in woodworking as a finish or an adhesive to produce a hard and durable protective layer over the substrate.
Polyurethane has many forms, such as solids, liquids, and sprays. These forms come in different colors, too, such as clear, amber, black, and red.
Polyurethane also comes in water-based and oil-based formulas. The water-based formulas are easier to apply because they dry faster than oil-based ones. They also tend to be more environment-friendly because they do not need toxic thinners.
Oil-based formulas have better durability, but they have disadvantages, too, such as yellowing over time and fumes that can give off a headache when applying them on woodworking projects indoors.
Polyurethane dries to become a hard, clear coat, so you can definitely sand polyurethane when it is completely cured.
Although it’s possible to sand polyurethane, it’s not always recommended. Remember that this is a clear coat, so any kind of abrasion of the surface would inevitably leave marks and cloud up the finish.
If your finish has some brush marks and textures, then sanding is possible, but you need to do it properly so that the final finish still looks clean and polished.
You can dry sand polyurethane, but there’s a much higher risk of scruff marks on the surface. Wet sanding, where you use sandpaper in combination with a thinner (either mineral spirits or water), is more recommended in this case.
We will tell you how to safely sand the final coat of polyurethane in a later section.
How Many Coats Of Polyurethane Is Enough?
One coat of polyurethane is not enough. It’s always recommended to apply 3-4 coats of polyurethane, but depending on the product that you use, this number might be different.
For example, if you’re using a water-based variation, make sure that you have at least five coats. This ensures that the polyurethane will provide the best possible protection. Oil-based polyurethane usually only requires 2-4 coats to work well.
As we’ve mentioned previously, you will need to wait until the polyurethane is completely cured to form a hard surface before it’s possible to sand. If you don’t wait for the finish to cure, sanding may create a big mess!
Depending on the product you use and how many coats you apply, the curing time might be different. Usually, it will take 1-3 days to cure completely.
How To Dry Sand Polyurethane
If you want to dry sand your polyurethane finish to get a smooth surface, here’s what you will need:
- Medium-grit sandpaper and fine-grit sandpaper
- Microfiber cloth
- A thinner (water or mineral spirits)
- Protective equipment (mask or respirator, gloves)
Once you have gathered the necessary tools, let’s get started.
1. Clean The Surface
The primary purpose of sanding is to get rid of minor imperfections. If there is dust or dirt on the polyurethane finish, those can create bumps that make it difficult to sand down, not to mention damaging to the surface if you try to sand it.
If the finish is too dirty, you may have to vacuum it up. If not, you can use a clean cloth to wipe down the polyurethane finish.
Afterward, use a damp cloth (soaked in a thinner – either water or mineral spirits) to wipe down the surface to get rid of the dust particles that may linger on the surface.
Water is a good thinner for both water-based and oil-based polyurethane finish, while the mineral spirit is better to use with oil-based polyurethane.
After wiping, you should wait for the liquid to evaporate completely before sanding.
2. Sanding With A Medium-Grit Sandpaper
You should wear a mask or respirator in this step because polyurethane dust is toxic and can cause health issues if inhaled. Remember that sanding is only needed to smooth out minor imperfections, so you should not sand down the entire surface.
In this step, you can use medium-grit sandpaper to sand down the textured areas that need to be smoothed out; make sure to go in the direction of the wood grain so that the sanding isn’t too obvious.
Medium-grit sandpaper is recommended in this step because it can remove the imperfections effectively while not causing too much abrasion on the final finish.
You should also go very light-handed in this step because it’s harder to smooth out scratches and fog when you are dry sanding.
After this step, if you are happy with the smooth surface, you can wipe the surface clean and leave it at that. If not, you can move on to step 3.
4. Sanding With A Fine-Grit Sandpaper
After step 3, if your polyurethane finish still has some texture from the first sanding, you can use fine-grit sandpaper to smooth out those marks.
Again, make sure to go in the direction of the grain to ‘hide’ the sanding nicely. It helps to go slow with controlled movements in this step so that you can achieve a smooth finish.
The purpose of sanding with fine-grit sandpaper is to help you smooth out the marks previously created by the medium-grit sandpaper, so you should not try to apply too much pressure here, as it will defeat the purpose.
How To Wet Sand Polyurethane
If you want to sand your polyurethane finish, wet sanding is a safer alternative. However, it’s still important to be careful since it can leave your work looking foggy.
Here’s what you will need to wet sand your polyurethane finish:
- Sanding sponge
- Medium-grit wet/dry sandpaper
- Microfiber cloth
- A thinner (water or mineral spirits)
- Protective equipment (mask or respirator, gloves)
1. Clean Your Surface
You can follow the same careful steps as above to clean your polyurethane finish before it’s ready for wet sanding.
2. Prepare Your Sanding Tools
If you have an oil-based polyurethane finish, you can use mineral spirits as the thinner for sanding. For water-based polyurethane, you can use water.
Pour your thinner into a container and use the sanding tools to soak up the liquid for about 15 minutes.
3. Wet Sand The Surface
Before you sand, pour a little bit of thinner on the surface around the area that you want to sand, and apply the sandpaper in circular motions to smooth out the imperfections.
The liquid will help smooth out the marks, so using medium-grit sandpaper is fine in this process.
You should occasionally wipe the surface with a clean rack to check the surface to see if the desired result is achieved. If not, you can keep going using the same method.
4. Dry & Buff
After wet sanding, you should leave the surface to dry on its own for about 24 hours. After it’s dry, you can use a microfiber cloth to buff the finish to create a shiny, glossy finish.
How To Apply Wax To Smooth Out Polyurethane Finish
Not sanding the final coat polyurethane is a common choice for many furniture refinishers and woodworkers. Sanding the final coat of polyurethane is rather risky, and you should not sand it (whether wet or dry) unless there is no other way.
If there are some marks or imperfections on the final finish, there’s still a way to get rid of them without having to sand the surface. You can use wax to buff out the imperfections to achieve a perfect finish.
Here’s what you will need if you choose to go with this method:
- 0000 fine steel wool
- Finishing paste wax of your choice
- Microfiber cloth
Here’s how to buff out imperfections using a finishing wax:
1. Clean The Surface
Use a microfiber cloth to wipe down the surface to remove any dust or dirt on the surface. Don’t worry if there are small dust particles that cannot be fixed with wax alone, so sanding is still needed.
2. Buff
After you’re happy with the wax finish, you can use a microfiber cloth to buff the finish to make it shiner.
Using wax is great because you can reapply it occasionally when you spot any scratches on the surface. It’s a quick and easy way to achieve a glossy, shiny finish without having to reapply the polyurethane.
Up Next: Can You Sand Wet Wood?
