What Happens If You Don’t Sand Between Coats of Polyurethane?

Ever thought a smooth finish could hide a myriad of sins?

The allure to forgo sanding between polyurethane coats is potent, yet the consequences are substantial. Achieving a flawless, glossy finish is the ultimate goal for any craftsman. But neglecting essential steps can lead to unwanted peeling and chipping

In this post, we are going to look at:

  • What Happens if You Don’t Sand Between Coats of Polyurethane?
  • Is it necessary to sand between coats of polyurethane?
  • Choosing the right grit for sanding polyurethane between coats.
  • Polyurethane products that do and don’t need sanding between coats
  • If it is possible to get a smooth finish without sanding between coats

Keep reading to uncover the vital importance of this often-skipped step.

Why sand between coats of polyurethane?

You should primarily sand between coats to smooth out dust nibs and remove imperfections. It’s also intended to help you create a stronger mechanical bond with the next coat if you apply the next coat after 24 hours.

Remember, fast-drying polyurethane and most water-based poly will remain glossy even after sanding, so it won’t necessarily help with adhesion.

But it’s still necessary to sand polyurethane between coats for a perfect finish.

Sanding polyurethane is typically done in two situations:

  1. Before applying the first coat: Sanding the surface before applying the initial polyurethane coat helps create a smooth and clean surface for better adhesion.
  2. Between coats: Sanding between coats of polyurethane removes imperfections, such as dust particles or brush strokes, and promotes adhesion for subsequent coats. It helps achieve a smoother and more professional finish.

Remember to follow the manufacturer’s instructions and allow the polyurethane to dry fully before sanding or applying additional coats.

Do you need to sand between coats of oil-based polyurethane?

Yes, it is generally recommended to sand between coats of oil-based polyurethane. Use fine-grit sandpaper to lightly sand the dried coat, creating a slightly rough surface.

Remove dust, apply the next coat, and repeat the process. This helps achieve a smooth finish and better adhesion between coats.

Do you need to sand between coats of water-based polyurethane?

Yes, sanding between coats of water-based polyurethane is crucial. It promotes adhesion and ensures a smooth, flawless finish by removing imperfections.

Use fine-grit sandpaper, around 220-grit, to lightly sand the surface once the first coat is dry.

Wipe away dust with a tack cloth before applying the next coat

Consequences of Neglecting to Sand Between Polyurethane Coats

Here are the consequences of not sanding between coats of polyurethane;

1. Dust and Debris Will Accumulate in Your Lower Coats

Dust and Debris on polyurethane floor

One of the reasons we lightly sand poly is to remove all the sanding dust and bubbles.

No matter what you do, there will always be bubbles when you first apply polyurethane.

However, most of the bubbles disappear within minutes. Whatever is left can be easily sanded. Dust particles, on the other hand, tend to come out of nowhere. Okay, that’s not one hundred percent accurate.

They can appear if you didn’t clean the wood floor properly before applying the polyurethane finish or dust blown into the room when you don’t dispose of the sanding dust properly.

But quite often, they appear.

In either case, sanding the bare wood again will get rid of both bubbles and dust nibs. If you don’t remove the dust nibs, your finished project will look dirty and remain that way no matter how often you try to clean it.

Needless to say, bubbles make your work look diseased and yuck! It will be a shame to let either of these things ruin all the money, time, and energy you have put into your project.

2. Subsequent Polyurethane Coats Won’t Adhere Properly

Polyurethane, especially oil-based polyurethane, is a unique product.

It’s not like paint, lacquer, or oil-based stains that don’t need to be sanded between coats. You see, poly is slippery and has a different makeup that prevents chemical solvents from bonding.

When the paint dries, you can apply another coat, and it sticks together. However, what happens with oil-based polyurethane is that the second coat sits on top of the first one like two cheese slices on a burger.

Sure, in some instances, they will fuse into each other, but not all the time.

Eventually, the topcoat will peel off. Lacquer, on the other hand, melts into each other, forming an inseparable bond. So the question is, do you want to risk your hard work peeling off after a few days or weeks?

3. Inability to Eliminate Brush Marks

Brush Marks on polyurethane floor

Another unsightly and painful error when applying polyurethane is leaving brush marks. But then, again, this has little to do with you as a user and more to do with the product that’s why you should look for fine foam brushes.

Polyurethane is a thick, heavy liquid, and it takes time to set or level out fully. As it levels, bubbles and brush marks tend to disappear. 

However, some companies are better at making polyurethane than others. As a result, some products will leave visible brush marks on the first and second polyurethane coating.

By the third coat, these imperfections are usually gone, although not by magic.

When you notice any brush marks, lightly sand over them and reapply a final coat of polyurethane. You might think that applying a subsequent coat of poly will cause the brush marks to disappear, but you’ll be wrong.

There is no substitute for sanding for some problems, and brush marks are one of them.

Another problem that can arise when applying too much polyurethane is cracking. This happens when it dries too quickly because of hot weather. This can either happen in between coats or when you are done.

Of course, it would be silly to apply another very thin coat without sanding if you notice this problem. Thankfully, it isn’t pervasive, so you may never face this problem.

The main advantage of sanding between polyurethane coats, besides the adhesion, is that it allows you to inspect your work and check for flaws.

3. Rough and dull finish

Not sanding between polyurethane coats can lead to a rough, dull finish. Sanding smooths the surface, removing imperfections like brush marks, bubbles, or dust.

These imperfections can occur during the application of the preceding coat. Sanding also creates tiny scratches, improving adhesion for the new coat. This leads to a more even, glossy layer.

Without sanding, the new coat may not bond well with the previous one. This can lead to poor adhesion and uneven coverage. A coarse or bumpy texture can develop.

Defects from the previous coat can become visible. They might also cause air pockets in the new coat.

The finish’s shine and clarity may also decrease without sanding. This is due to the layer accumulation, affecting light transmission and polyurethane oxidation.

5. Chipping, peeling, cracking of the Finish

Not sanding between polyurethane coats can make the finish more susceptible to chipping, peeling, and cracking. This happens because unsanded layers might not integrate well.

They can react differently to changes in temperature and humidity.

Sanding creates a stronger bond between polyurethane layers. This makes the finish more resistant to damaging external factors. Without sanding, the layers might adhere weakly. They may separate or detach from each other or the substrate over time.

The finish could also crack or peel due to the stress from the expansion and contraction of the wood or the polyurethane.

Hence, sanding between coats can prevent these issues. It ensures a durable and stable finish for your project.

Chipping, peeling, and cracking of the polyurethane Finish

Opinions About Not Sanding Polyurethane Between Coats

When preparing to write this article, I wanted to find out what already exists on the internet on the question, “What Happens If You Don’t Sand Between Coats of Polyurethane” and what I saw was very disturbing.

Many websites said “nothing serious” will happen if you don’t want sand polyurethane coats.

Well, if “nothing serious” will happen, why have we been sanding between coats all these years? Why bother spending hours of your time doing something without severe consequences? Heck, why even have a conversation about it at all?

But, you know what? I will keep my cool and tell it as it is (deep breaths).

Okay, regarding this question, here are some things that can go wrong if you fail to sand between coats of polyurethane lightly.

Can you recoat polyurethane without sanding?

No, you generally cannot recoat polyurethane without sanding.

Sanding is essential when applying a new coat of polyurethane over an existing cured one. It promotes proper adhesion, prevents peeling, chipping, and ensures the longevity of the finish.

Without sanding, the fresh coat may not adhere well to the cured surface, leading to potential durability and finish issues. Before recoating, lightly sand the old surface for optimal results.

How to sand between polyurethane coats

Supplies Needed:

  • Fine-grit sandpaper (220-320 grit)
  • A sanding block or pad (optional)
  • Tack cloth or a soft, lint-free cloth
  • Dust mask
  • Protective eyewear
  • Vacuum or brush (for cleaning up sanding dust)
  • A well-ventilated workspace
  • Polyurethane (for the next coat)
  • Paintbrush or foam brush (for applying polyurethane)

Step 1: Preparation

Ensure you are in a well-ventilated space and wear protective eyewear and a dust mask. Allow the polyurethane coat to dry completely, following the drying time recommendations on the product label.

Step 2: Sanding

Once the coat is completely dry, fold the fine-grit sandpaper around a sanding block or pad to maintain an even surface.

Lightly sand the entire surface in the direction of the grain. Be careful at edges and corners.

Step 3: Cleaning

After sanding, remove all sanding dust from the surface using a vacuum, brush, or a tack cloth.

Wipe the surface with a lint-free cloth to remove any remaining particles.

Step 4: Applying the Next Coat

Apply a thin, even coat of polyurethane using a clean brush or foam brush. Allow it to dry as per the manufacturer’s instructions.

Step 5: Repeat as Necessary

If more coats are needed, repeat the sanding, cleaning, and application steps. Allow each coat to dry thoroughly before proceeding to the next one.

What Grit Sandpaper to Use Between Coats of Polyurethane?

The aim of sanding between only two coats is to help it adhere better and not distort the previous one, so you need to sand lightly so as not to create scratches.

For most projects, 320 sandpaper grit will suffice. However, you may also use finer grits like 400 or clogging sandpaper, and the result will still be pristine.

Regarding woodworking tools, nothing beats the comfort of sanding by hand. However, if you’ve got a large project or you don’t want to wait too long between poly coats, then electric sanders will come in handy.

In case you are working on a large deck, here’s the grit sandpaper to sand a deck.

The best sandpaper for polyurethane

  1. 3M SandBlaster Sandpaper: 3M is a reputable brand known for producing high-quality sandpaper. Their SandBlaster line offers fine-grit sandpaper suitable for finishing and smoothing polyurethane surfaces.
  2. Norton ProSand Sandpaper: Norton is another trusted brand that manufactures a range of sandpaper products. Their ProSand line includes fine-grit sandpaper designed for smooth finishing, making it a good choice for polyurethane sanding.
  3. Mirka Gold Sandpaper: Mirka is known for producing abrasives specifically for woodworking and finishing applications. Their Gold series sandpaper offers fine-grit options suitable for sanding polyurethane.

Can I use an orbital sander between coats of polyurethane?

Using an orbital sander between coats of polyurethane is generally not recommended. Orbital sanders are more aggressive and can remove too much material, potentially damaging the previous coat or the wood surface.

It is best to use a fine-grit sandpaper by hand to lightly sand between coats of polyurethane. This allows for more control and reduces the risk of over-sanding.

Do you have to sand between coats of polyurethane?

Yes, sanding between coats of polyurethane is generally required. It’s crucial for adhesion and a smooth finish.

However, alternative approaches are available for those seeking to bypass this step, offering convenient options to achieve a polished look without sanding.

1. Use Unique Products That Don’t Require Sanding

As we’ve mentioned, some poly products don’t require sanding. These are usually water-based polyurethane. You can check our best water-based polyurethane for floors review if you plan to refinish your smooth surface soon.

You might also find oil-based finishes that don’t require sanding and give decent results.

2. Don’t Use a Gloss Finish

For oil-based polyurethanes, don’t use a high gloss finish; instead, use semi-gloss. Gloss does not have curing agents like semi-gloss polyurethane formula or satin finishes.

As a result, flaws show up more, and subsequent multiple coats don’t adhere well without sanding.

On the other hand, Satin can be applied with minimal repercussions, without sanding. 

3. Use One Coat Poly

Another option is to use a product that only requires one coat: no sanding, no next coat, no waiting. And on top of that, they also usually dry pretty quickly, despite being oil-based.

I will be remiss if I don’t add another disclaimer here. One coat poly can be somewhat problematic. For starters, they are usually very thick, so they don’t level out well. Additionally, because they dry quickly, they tend to show brush strokes.

Personally, I don’t use it, and I also don’t recommend it for beginners.

However, if you`re a professional, working on a small area, or don’t mind a bit of experimentation, feel free to test any of these products and share your results with me.

How Long to Wait Between Polyurethane Coats

It’s crucial to wait for each coat to dry before applying more polyurethane. The drying time depends on the polyurethane type and brand; oil-based ones take 12-24 hours, while water-based can dry in 6-12 hours.

Some brands offer fast-drying variants, allowing recoating in even shorter times, but these require care, especially for beginners.

Mistakes made with fast-drying products are less forgiving and harder to correct.

Additionally, they are pricier, so errors can be costly. While slower, the conventional approach is safer and more forgiving of errors.

FAQs

Can you sand polyurethane?

Yes, you can sand between polyurethane coats, but not the final coat. Allow at least 24 hours for the first coat to dry and cure before sanding.

With a fine-grit sandpaper, gently sand along the direction of the wood grain. Remove the dust with a static duster before finishing with a tack cloth.

Should I sand between coats of Minwax polyurethane?

Yes. Minwax Polyurethane should be sanded between coats. Sand with 320 grit sandpaper to get rid of any fine particles of dust that have settled on it while it was still wet.

Sanding, in this case, not only allows for a smoother finish but can also help abrade the surface and increase inter-coat adhesion.

For amazing results, use paste wax over polyurethane, which will further enhance the protection and luster of your finish.

Can you wet sand polyurethane?

Wet sanding polyurethane is generally not recommended. Polyurethane is a type of finish that is typically sanded dry. Wet sanding involves using water or a lubricant to reduce friction and create a smoother sanding experience.

However, water can cause the polyurethane finish to become cloudy or dull. It is best to follow the manufacturer’s instructions and sand polyurethane using traditional dry sanding methods with fine-grit sandpaper.

This will help you achieve a smooth and professional finish without compromising the appearance of the polyurethane.

Sanding between coats of water based polyurethane

Because we need at least 3 coats of water-based polyurethane to get a smoother and more professional finish, you must sand between coats.

Sanding is not required between coats for adhesion, especially if each coat dries in less than 6 hours. But, we don’t recommend your recoat polyurethane on the same day.

Do you have to sand between coats of spar urethane

It depends. Within 10 – 12 hours, you can recoat Spar urethane without sanding. Beyond 24 hours, sand with 220-320 grit paper to remove brush strokes and bubbles but not help the urethane polymers stick.

When applying several coats of urethane, brush along the wood grain, spreading it into as thin a coat as possible. Don’t scuff sand the last coat.

Do you have to sand between coats of polyurethane on floors?

Yes, it is generally recommended to sand between coats of polyurethane when refinishing floors. Sanding between coats helps to ensure a smooth and even finish, removes imperfections, and promotes better adhesion of subsequent coats.

It allows for proper bonding between the layers of polyurethane, resulting in a more durable and professional-looking floor finish.

Proper sanding techniques and appropriate grit sandpaper are important for achieving the desired results.

Related Reading: What kind of polyurethane to use on hardwood floors

Do You Have to Sand Between Coats of Polyurethane?

Despite all the fancy products on the market, nothing beats the results you get when you sand between polyurethane coatings.

Yes, it takes time and a bit of money, but you know what happens if you don’t sand between coats of polyurethane.

Plus, it is the only way to quickly correct errors and prevent you from having to start all over.

But hey, if you have used poly without sanding, please let me know.

We’ve also responded to the question, “Do you have to sand the final coat of polyurethane? Check it out, as the consequence of rushing is that you risk compromising all your work.

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