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?
There are two ways on how to get a smooth polyurethane finish. The first is dry sanding the final coat, which is what most people are familiar with. It involves only the use of sandpaper.
The other type is 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 end up leaving scuff marks on the polyurethane. Remember that sanding scratches the surface of the finish. If you use anything higher than 600-grit sandpaper, the final coat could end up looking 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, make sure there are enough coats of polyurethane on the surface. If you have only one coat, any type of sanding method will get to the bare wood, and you’ll have to apply another coat.
How Many Coats of Polyurethane?
If you are using water-based polyurethane, make sure 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 you 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 oil-based polyurethane, 4-5 coats are also advisable. However, if the product you are working with only requires 2 coats, then sanding the third coat should be just fine.
Also, make sure the surface has cured fully before you start this process. If not, you risk abrading much more of the coat than you would expect and leave a mess.
Now, let us begin with the method for dry sanding.
Tools You’ll Need for Dry Sanding
- 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 get rid of any dust particles lying around. 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 be left with a few dust particles hanging around. To get rid of 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 were able to 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. If you didn’t use a lot of liquid, it shouldn’t take too long. 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 are still there, then it’s time for the next step.
Step 4: Start Sanding.
Before you begin sanding, put on your respirator or face mask. Wood dust can cause different health problems, so don’t take any chances, even with a 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 in order not to 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 really 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.
How to Wet Sand the Final Coat of Polyurethane
As mentioned before, wet sanding is the safer option of the two, as it is less likely to leave your work cloudy or scratched. However, you still need to be careful, so you don’t end up abrading too much of the floor.
Tools You’ll Need for Wet Sanding to Fix Uneven Polyurethane Finish
- 400-grit wet/dry sandpaper
- Sanding sponge
- Vacuum cleaner
- Tack cloth
- Mineral spirits or water
Steps to Wet Sanding the Last 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 on the Surface and Start Sanding
Pour a small quantity of water at a time and begin to 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 need to 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 cloths. Instead, get a fresh one each time you are wiping the floor.
If the particles you are trying to remove are still there, 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.
What Happens if I Don’t Sand the Final Coat of Polyurethane?
Nothing happens if you don’t sand the final coat of polyurethane. Therefore, you should only sand the last coat if you notice a few 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, which can ruin your work if you’re not careful. Even when using wet/dry sandpaper, you need to be careful not to rub too much of the coat.
Buff Polyurethane Finish When Necessary
Back to your question: Do You Sand the Final Coat of Polyurethane? Well, sanding the final coat is not necessary and is also a rather risky process. When not done correctly, you will need to sand and recoat the entire surface.
If you must sand the final coat, the wet sanding method is safer and gives a smoother or glass-like finish on the wood.
The best solution is to ensure you get the polyurethane application right the first time, so you don’t have to go through this trouble.