Believe it or not, applying polyurethane coats on a wood surface doesn’t automatically yield a smooth finish. Because of this, you may surely wonder if you need to sand the final coat of polyurethane.
So rather than blindly pushing your wood projects into the sanding process, our expert woodworkers took the liberty to discuss how you should handle this matter.
Is Sanding Required for the Final Coat of Poly?
If the final coat of polyurethane dries with a smooth finish, sanding the entire surface is unnecessary. However, if you see the material with obvious signs of a rough finish, it’s a step we don’t recommend skipping.
You must consider this process if you spot brush marks, bubbles, or dust nibs settled in the material after its last coat has dried. Otherwise, these obstructions will get in the way of getting a glossy and durable coating.
What You Need to Know About Polyurethane
Like other wood finishes, polyurethane application is meant to give maximum protection on a bare wood surface. It’s a durable varnish formula made of tiny resin molecules.
If applied properly, these components are meant to result in a smooth finish and add an extra layer of resistance against water exposure and abrasion.
You’ll most likely encounter two variations when you look for a polyurethane finish in the nearest hardware stores.
Here’s more information about oil-based and water-based polyurethane products to help determine which is best for your wood project.
Water-Based Polyurethane and Its Pros and Cons
True to its name, water-based polyurethane is a wood finish with water as its base solvent. And because water carries its polyurethane solids, it yields a clearer final coat.
Pros
- Dries faster than oil-based polyurethanes
- Easy cleaning process with warm water and clean cloth
- Environmentally friendly with no unbearable odor
- You can apply this polyurethane finish indoors
Cons
- Requires more coats to form durable layers
- More susceptible to wearing & tearing
Oil-Based Polyurethane and Its Pros and Cons
Meanwhile, oil-based polyurethane options include organic solvents as their base, like petroleum and minerals. Although the oil-based polyurethane finish is thicker than water-based alternatives, they’re easier to apply and offer more glossy results.
Pros
- More durable final coat than water-based polyurethane
- Requires fewer coats to yield a smooth finish
- More gloss effect after the last coat dries
Cons
- It has a slow drying time
- Prone to marks and dust bumps
How To Smoothen the Final Polyurethane Coat: 2 Methods
Method #1: Dry Sanding The Final Coat
Dry sanding only requires sandpaper, so it’s a process most woodworkers are familiar with. However, using this method on a polyurethane finish can lead to potential surface damage, like scuff marks and scratches.
Before you sand the final coat of polyurethane, check if the material has been applied with enough coats to withstand the dry sanding process. If not, sanding polyurethane on the first coat will expose the surface’s bareness.
Tools You’ll Need
- Sanding Block
- 600-grit dry sandpaper / Sanding Tool
- Vacuum Cleaner
- Water/Mineral Spirits
- Reliable respirator
- Tack Cloth
Step #1: Vacuuming the Surface
Since you’re sanding polyurethane finish to eliminate minor imperfections on the surface, you wouldn’t want dust nibs and bumps settling on the material as you do this process.
You must free the surface of any obstructions that may cause a noticeably uneven finish with a vacuum cleaner.
Don’t forget to run the tool through every square inch of the area if you’re working on hardwood floors, especially the hard-to-reach corners. These areas should be free of wood dust particles to ensure better dry sanding results.
If you’re trying to dry sand smaller workpieces like desks, our resident woodworkers urge you to use a tack cloth instead of this tool.
Step #2: Wipe Off the Surface with Water or Mineral Spirits
Vacuuming may not eliminate all the dust particles, so the best you can do is wipe them off with a water-soaked lint-free rag. If you’re sanding a surface already coated with oil-based polyurethane, remember that you also need to pour mineral spirits on the cloth.
Step #3: Allow the Water to Evaporate
Let the surface sit and dry for just a little bit before sanding the final coat of polyurethane, just like sanding between coats of stain. Once the moisture from the water evaporates, you can check if the slight imperfections are gone. If not, you can start sanding on the next step.
Step #4: Begin Sanding
No matter what sanding tools you’ll use (even fine-grit sandpaper), don’t skip wearing a respirator or mask because these wood particles can pose potential health problems [1].
Attach the 600-grit sandpaper to a sanding block or any sanding tool at your disposal and lightly sand the surface. As you hover through the parts with dents or bubbles, it’s wise to sand with the wood grain to avoid blotched results.
Step #5: Clean the Surface and Use a Finer Grit
After you sand the final coat of polyurethane finish, grab the tack cloth and wipe the surface clean. If unsatisfied with the coat’s smoothness, you can sand polyurethane with finer sandpaper.
Method #2: Wet Sanding The Final Coat
If you’re not a fan of having your final coat at risk of scratches, the best you can do is execute the wet sanding process. However, you must practice caution because wet sanding can lead to abrasion on the surface.
Before you wet sand the final coat, don’t forget to check how many coats of polyurethane are applied on the surface.
Tools You’ll Need
- Tack Cloth/Lint-free Cloth
- Water/Mineral Spirits
- Steel Wool
- Paste Wax
- Sanding Sponge
- Vacuum Cleaner
- 400-grit wet/dry sandpaper
Step #1: Follow the First Three Steps of Dry Sanding
You should follow the first three initial steps of dry sanding to execute this method.
Step #2: Pour Water or Mineral Spirits into Your Cup
After that, pour around a quarter-inch mineral spirit into a container or a cup. You can substitute it with water. However, mineral spirits guarantee higher chances of getting better results after sanding the final coat of polyurethane.
Step #3: Dip Your Sanding Sponge or Dry/Wet Sandpaper in Water
Place the sandpapers or sanding sponges on the container full of mineral spirits and let it soak for at least fifteen minutes.
Step #4: Drop Water on the Surface and Begin Sanding
If you soak the sandpapers for 24 hours, you don’t have to pour the mineral spirits over the final coat of polyurethane finish. Otherwise, you’ll have to add the liquid.
As you sand the final coat, guide the sanding tool into a circular pattern. By doing this, the surface won’t get cloudy.
Step #5: Wipe Off the Surface and Allow it to Dry
Adding water or spirits on the surface can obstruct your sight, so we suggest wiping it with a cloth as you continue to sand. Don’t use the same rag in every wipe because it would just put the dust back in the sanded spot.
Leave the surface to dry for 24 hours. Once that’s done, you’ll need to buff the material with a dry rag.
How to Buff Poly for a Smoother Finish
Whether you sanded the last coat of polyurethane or not, you must buff the surface with steel wool and wax to ensure that it’ll yield a smooth finish. Here’s how you can do it:
- Find steel wool with 0000 configurations and coat it with paste wax.
- Apply it back and forth in the grain’s direction.
- Grab another piece of 0000 steel wool and buff the remaining imperfections out.
Polishing Your Final Poly Coat: 5 Extra Tips
Tip #1: Mark the Surface with a Pencil
Add pencil lines on the final coat of polyurethane because this could determine the right sandpaper grit you should use for sanding.
Tip #2: Keep Your Working Station Clean
Don’t forget to clean the material and the surfaces surrounding it. Through this, you can achieve the desired finish faster without the delay of eliminating dust particles.
Tip #3: Make Sure to Wipe Wood
Although we mentioned that you could use water to soak the cloth, it’s not a method we’re keen on for wiping wood materials. You may not know, but it’ll lift the surface up. When that happens, we suggest you sand the final coat of polyurethane again.
Tip #4: Utilize the Right Tools and Materials
We know that sanding can be a tedious job. So if you don’t want to end up doing it, ensure that you’re applying poly with the right tools and materials.
If you’re working on large-scale surfaces, your final coat of polyurethane should be applied with rollers. And if it’s smaller pieces with complex corners and joinery you’re handling, utilizing a foam brush should suffice.
Tip #5: Use Sandpaper and Pads
You’ll encounter stubborn dust nibs that won’t yield with fine-grit sand pads, so our best suggestion is to use ones with 280 grit.
How Many Poly Coats Do You Need to Apply?
The surface should form enough protective layers when you apply polyurethane upon its third coat. It should seal the surface well to stand strong against any sanding procedure.
Can You Not Touch the Final Coat of Poly?
Most woodworkers get the initial coats sanded for extra adhesion, but they don’t sand the final coat of polyurethane finish. This case is especially true if you’re not aiming for an ultra-smooth surface.
Will Sanding Poly Affects Your Projects Outcome?
As long as you don’t mitigate the causes of dust and bumps on the surface, deciding not to sand the final layer of polyurethane won’t affect your project much. You can also skip this step if the material only shows unnoticeable imperfections.
Suggested Readings:
What Will Happen if You Don’t Sand Between Poly Coats?
Some may think sanding every polyurethane coat can be troublesome, but did you know that it has serious repercussions if you skip it? Here are the risks you’ll expose your project to.
Durability
Apart from the part where you sand the final coat, sanding between polyurethane applications helps the surface to form better adhesion for the next coating. Through this, the material will be more durable. For outdoor applications, check this review of the best exterior polyurethane for your project!
Finish
It’s not required to sand the final coat and the previous polyurethane applications. However, doing this method will yield better finish results.
Also Read: Is Sanding Between Coats of Paint Necessary?
FAQ
Can you use dry sandpaper to do wet sanding?
It’s not recommended to use dry sandpaper for wet sanding because it’s susceptible to clogging. If you’re not cautious, this material can ruin your project.
Conclusion
So, do you need to sand the final coat of polyurethane? Yes, it’s advisable, especially for rough surfaces.
Though you may think skipping it altogether is more reasonable, but trust our resident woodworkers when we say it’ll come in handy when you need to polish a material.
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