It seems like the more you learn about polyurethane, the more complicated it seems. With many conflicting information, different hacks, and new products, it is hard to keep things straight.
In this article, we’ll tackle:
- Do you need to sand the final coat of polyurethane
- How to lightly sand polyurethane for a smoother finish
- What happens when you do or don’t sand the final coat of polyurethane
- How to get bumps out of polyurethane finish
Now, let’s jump straight to it.
Do You Sand the Final Coat of Polyurethane?
No, you don’t need to sand the final coat of polyurethane. The only time you should sand the final coat of polyurethane is if you haven’t gotten a smooth finish. Different problems can occur when you apply polyurethane, such as dust nibs, bubbles, and streaks.
If any of these is visible, the job isn’t complete.
How Do You Sand and Smooth the Final Coat of Polyurethane?
Okay, maybe we should start with “Can you sand polyurethane?”
Definitely, YES! And there are two ways to get a smooth polyurethane finish through sanding.
- Dry sanding the final coat is what most people are familiar with. It involves only the use of sandpaper.
- Wet sanding, which uses a combination of wet/dry sandpaper and mineral spirits or water.
Dry sanding isn’t the best option as it could leave scuff marks on the polyurethane. Remember that sanding scratches the surface of the finish.
If you use anything above 600-grit sandpaper, the final coat could look cloudy and scuffed.
However, if you must use dry sanding, we will show you how to do it safely.
Before you start either of these, ensure there are enough polyurethane coats on the surface. If you have only one coat, any sanding method will get to the bare wood, and you’ll have to apply another coat.
Also, read our article on comparing sanding sealer with polyurethane.
How Many Coats of Polyurethane?
Is one coat of polyurethane enough? Are 4 coats of polyurethane enough?
If you use water-based polyurethane, ensure you have at least 5 coats, although some people recommend 7 or 8.
Most top-quality water-based polys work best with 3 or 4 coats, so if you have a fifth coat and sand it slightly, you’ll still get maximum protection.
Of course, if you had to apply more coats because the polyurethane raised the grain of the wood, then add an extra layer or two before sanding.
For non-yellowing oil-based polyurethane, 4-5 coats are also advisable. However, if the product you are working with only requires 2 coats, sanding the third coat should be fine.
Also, ensure the surface has cured fully before starting this process. If not, you risk abrading much more of the coat than expected and leaving a mess.
Now, let us begin with the method for dry sanding.
Tools You’ll Need for Dry Sanding Polyurethane Final Coat
- 600-grit sandpaper
- Sanding block
- Vacuum cleaner
- Tack cloth
- Mineral spirits or water*
How to Dry Sand the Last Coat of Polyurethane
Step 1: Vacuum the Surface
The main reason for sanding the final coat is to get rid of bumps, dust nibs, and other minor imperfections. It is then critical to ensure there is nothing that can cause bumps and an uneven polyurethane finish.
A vacuum cleaner will clean the wood after sanding. It is important to hover across every inch of the space, particularly any crevices in the corners if you are working on a floor.
If you are working on something much smaller, like a chair or a desk, you can use a tack cloth instead of a vacuum cleaner.
Step 2: Wipe the Surface with Mineral Spirits or Water
After you vacuum the floor, you might still have a few dust particles hanging around. To remove those, soak a lint-free cloth in water for water-based polyurethane and mineral spirits for oil-based polyurethane, then wipe the floor.
It is okay to use water for both.
This process may not be necessary if your vacuum cleaner did a great job and you could use the crevice tool and other bells and whistles. However, it doesn’t hurt to be safe.
Be careful not to drench the floor in liquid. Just a little bit will suffice. If you notice any dust on the cloth, discard it and use a new one.
Step 3: Let the Water Evaporate
Give the surface time to dry again. It shouldn’t take too long if you don’t use a lot of liquid. When it evaporates, inspect the surface to see if there are still any imperfections to correct.
There are times that dust on the surface makes the poly look bad. If the problems remain, it’s time for the next step.
Related read: Steps to resolve the issue of polyurethane not drying.
Step 4: Start Sanding
Before you begin sanding, put on your respirator or face mask. Wood dust can cause health problems, so don’t take any chances, even with fine-grit sandpaper.
Put the 600-grit sandpaper on a sanding block and gently swipe at the part of the finish with the dents, bubbles, or dust nibs. You want to do this gently to not tamper with the smooth parts of the finish.
The sanding block will help you be more accurate. As usual, you should sand with the grain. 600-grit is fine, but you should still be extra careful so the finished work doesn’t look cloudy or blotched.
Step 5: Clean the Surface
Use a tack cloth and wipe the surface. Check to see if the imperfections are gone. If they are, then your work is done.
If it looks better but still not as smooth as you would like, move on to step 6.
Step 6: Use a Finer grit
Move on to a finer grit, using the same technique as before. No matter how tempting it is, don’t sand against the grain, even if you feel like it will get the dust nibs or dents out better.
Repeat steps 5 and 6 until you get a smooth finish.
Can you wet sand polyurethane?
Wet sanding polyurethane has advantages over dry sanding, minimizing dust particles and preventing sandpaper clogging. It can achieve a smoother finish by using water as a lubricant.
However, it’s not always necessary, best for final coats or refinishing, and should be done after the polyurethane has cured. Proper techniques and appropriate grit sandpaper are essential to avoid damaging the finish or creating uneven patches.
How to Wet Sand the Final Coat of Polyurethane
Wet sanding is safer, as it is less likely to leave your work cloudy or scratched.
However, you must avoid abrading too much of the floor.
Tools You’ll Need for Wet Sanding Polyurethane Final Coat
- 400-grit wet/dry sandpaper
- Sanding sponge
- Vacuum cleaner
- Tack cloth
- Mineral spirits or water
Steps to Wet Sanding the Final Coat of Polyurethane
Follow steps 1-3 of dry sanding.
Step 4: Pour Mineral Spirits or Water into a Cup
Pour about a quarter-inch of mineral spirits or water into a cup. You should only use mineral spirits for oil-based. However, you can use water even for oil-based polyurethane and still get good results.
Step 5: Soak the Wet/Dry Sandpaper or Sanding Sponge in the Water.
Immerse your sanding tool in the water until it soaks up a fair amount. You can either soak it for 24 hours, in which case you won’t need to pour water on the surface of the polyurethane, or soak it for fifteen minutes.
Step 6: Pour Water onto the Surface and Start Sanding
Pour a small quantity of water at a time and rub the surface with the wet/dry sandpaper. Sand the imperfections in a circular pattern. The water will prevent the polyurethane from becoming cloudy and keep the sandpaper light’s scratch marks.
Step 7: Wipe the Surface
Water will obstruct your view, so you must get rid of it often. Constantly wipe the surface with a clean cloth to properly assess your progress. Just as when cleaning the floor normally, don’t repeat clothes. Instead, get a fresh one each time you are wiping the floor.
If the particles you try to remove are still the, repeat steps 6 and 7 until you get the desired result.
Step 8: Leave it to Dry, then Buff with a Dry Rag
Once you’ve gotten the imperfections out, leave the surface to dry for 24 hours.
After that, use a lint-free rag and buff the surface. The surface will be shiny once again and smoother than ever.
Interesting read: What causes bubbles in polyurethane?
What Happens if I Don’t Sand the Final Coat of Polyurethane?
I know you are wondering, “Do I have to sand between coats of polyurethane? What happens if I don’t?”
Well, nothing happens if you don’t sand the final coat of polyurethane. Therefore, you should only sand the last coat if you notice minor problems.
However, if there are bubbles, the surface is noticeably uneven, or there are many dust nibs, you should sand between coats of polyurethane as usual and apply another coat.
Can I Use Dry Sandpaper for Wet Sanding?
No, you should not. Dry sandpaper will get clogged quickly, ruining your work if you’re not careful. Even when using wet/dry sandpaper, you must be careful not to rub too much of the coat.
What grit sandpaper for final coat of polyurethane?
A grit size between 220 and 400 is great for the last coat. This fine grit helps to remove imperfections, dust particles, or unevenness that may have occurred during the previous coats of polyurethane or in the drying process. It also creates a surface that promotes better adhesion and a smoother finish for the final coat.
Recommended read: What will remove polyurethane from wood without sanding?
How to buff the final coat of polyurethane
To buff the final coat of polyurethane and achieve a smooth and glossy finish, follow these general steps:
- Ensure proper curing: Allow the final coat of polyurethane to cure. This usually takes several days.
- Clean the surface: Before buffing, clean the surface to remove any dust, debris, or contaminants. Use a soft cloth or a tack cloth to wipe the surface gently.
- Choose a buffing method: You can use several methods to buff polyurethane. The two most common methods are hand buffing and machine buffing.
- Hand buffing: Apply a small amount of polishing compound or fine abrasive polish to a soft, lint-free cloth. With light to moderate pressure, rub the cloth in circular motions over the polyurethane surface. Continue buffing until the desired level of shine is achieved. Wipe away any excess polish with a clean cloth.
- Machine buffing: If using a machine buffer or polisher, attach a foam or polishing pad suitable for polishing finishes. Apply a small amount of polishing compound to the pad. Work the machine over the surface using slow, overlapping passes. Be cautious not to apply too much pressure or stay in one spot for too long, as it can remove too much polyurethane. Clean off any residue with a soft cloth.
- Inspect and repeat if necessary: After buffing, inspect the surface for any remaining imperfections or unevenness. You can repeat the buffing process with a finer polishing compound or use a finer-grit foam or polishing pad.
- Final cleaning: Once satisfied with the buffed finish, clean the surface again to remove any residue from the buffing process. Use a clean or tack cloth to remove any polishing compounds or debris.
Read also: What is the time gap for polyurethane coats on hardwood floors?
Final thoughts on do you sand polyurethane after the final coat
Back to your question: Do You Sand the Final Coat of Polyurethane? Well, sanding the final coat is unnecessary and a rather risky process. When not done correctly, you must sand and recoat the entire surface.
If you must sand the final coat, the wet sanding is safer and gives the wood a smoother or glass-like finish.
The best solution is to get the polyurethane application right first so you don’t have to go through this trouble.
Read the battle of polyurethane vs spar urethane to know which is best for you and your projects.