You might think that just slapping on a coat of polyurethane is going to give you a smooth finish, but that’s not always the case. I’ve been there, questioning whether I should sand that final coat or not.
So, instead of just diving into the sanding process with uncertainty, I’m here to share some expert advice on how best to handle this situation.
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 I do not 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
Polyurethane, like other wood finishes, is designed to provide maximum protection to bare wood surfaces. It’s made up of tiny resin molecules and is a pretty durable varnish formula. I often use it in my projects to give them that extra layer of protection.
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
Water-based polyurethane lives up to its name—it uses water as its base solvent. I’ve noticed that because the polyurethane solids are carried in water, you end up with a much clearer final coat. It’s a go-to for projects where clarity is key.
Pros
- It dries faster than oil-based polyurethanes
- The cleaning process is easy with warm water and a clean cloth
- It's considered environmentally friendly with no unbearable odor
- You can apply this polyurethane finish indoors
Cons
- It requires more coats to form durable layers
- It's 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
- It's a more durable final coat than water-based polyurethane
- It requires fewer coats to yield a smooth finish
- It has a more gloss effect after the last coat dries
Cons
- It has a slow drying time
- It can be prone to marks and dust bumps
How To Smoothen the Final Polyurethane Coat: 2 Methods
Method #1: Dry Sanding The Final Coat
Dry sanding is a process that many woodworkers are quite familiar with, involving sandpaper. However, using this method on a polyurethane finish can be risky.
It might end up causing surface damage, leaving behind scuff marks and scratches. It’s worth being cautious about this when working with polyurethane finishes.
Before you sand that final coat of polyurethane, make sure you’ve applied enough coats for it to stand up to dry sanding. I’ve learned that if you jump in too soon and sand after just the first coat, you’ll likely expose the raw surface underneath. Proceed with caution with this method:
Tools You’ll Need
- Sanding lock
- 600-grit dry sandpaper or a 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, I suggest you to use a tack cloth instead of this tool.
Step #2: Wipe Off the Surface with Water or Mineral Spirits
Even after vacuuming, you might still have some pesky dust particles lingering around. In my experience, wiping them off with a water-soaked, lint-free rag works like a charm.
And here’s a tip: if you’re sanding a surface that’s coated with oil-based polyurethane, don’t forget to dampen the cloth with mineral spirits before wiping. It ensures you get that clean, dust-free finish you’re aiming for.
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. (Know if it’s really advisable to sand wet wood here!)
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
Once you’ve wiped off the dust, grab a container or cup and pour in about a quarter-inch of mineral spirits.
While you can use water as a substitute, I’ve found that mineral spirits offer better results when it comes to sanding that final coat of polyurethane. It’s a step that can really make a difference in the outcome.
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 I recommend 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
Before you dive into sanding that final coat of polyurethane, take a moment to add some pencil lines on the surface. It might seem simple, but these lines can actually help you figure out the ideal sandpaper grit for the job.
It’s a trick I’ve used countless times to ensure I’m using the right grit and getting the results I want.
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 I mentioned that you could use water to soak the cloth, it’s not a method I’m keen on for wiping wood materials. You may not know, but it’ll lift the surface up. When that happens, you can sand the final coat of polyurethane again.
Tip #4: Utilize the Right Tools and Materials
I 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 my 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 Project's 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.
Therefore, it is essential to approach sanding polyurethane with proper knowledge, skill, and attention to detail to achieve the desired outcome. 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.
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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 me when I say it will come in handy when you need to polish a material.
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