What To Do If The Final Coat Of Polyurethane Feels Rough

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The finishing part of a woodworking project has many options, with staining and painting being the most obvious.  It’s the choice of topcoat that has many more options, and one of the most common today is polyurethane.

You’ve likely worked with polyurethane before and have some idea of the process of application.  Skipping steps in that process, though, can lead to something much less than a smooth finish.

Key Points:

  • Don’t worry – it’s not fatal.  There are cures, and they are not onerous and will attend to brush marks, bubbles, dust nibs, bugs, and other symptoms of a rough finish.
  • A wood pre-treatment, a solid surface clean, sanding between coats of poly, a bit of sanding or steel wooling, can all be effective in creating a smooth finish.
  • Thin and even coats, letting each polyurethane coat dry fully, smoothing between them, could help avoid that rough feel on a final coat.

However, there are some steps you can take to avoid this, as well as steps you can take to cure it if the final coat of polyurethane is rough.  We can help with this, and it is not onerous or burdensome in any way.

Neither does it require any special tools.

The Two Types of Polyurethane

Water-Based Polyurethane

To understand what might be causing that rough feeling after applying your poly, it’s important to know the different types of polys.

We have two main types of polyurethane, water-based and oil-based, to choose from:

  • Water-based polyurethane is a clear, colorless finish that is easy to apply and dries quickly. It is a good choice for indoor applications, as it is low-odor and non-toxic. However, it is not as durable as oil-based polyurethane and may not be as resistant to scratches and chemicals.
  • Oil-based polyurethane is a yellowing finish that is more durable than water-based polyurethane. It is a good choice for outdoor applications, as it is more resistant to moisture and UV rays. However, it is more difficult to apply an oil-based poly, and dries more slowly than water-based polyurethane.

Here is a table summarizing the key differences between water-based and oil-based polyurethane:

FeatureWater-based PolyurethaneOil-based Polyurethane
ColorClearYellowing
OdorLowHigh
ToxicityNon-toxicToxic
DurabilityLess durableMore durable
Resistance to moistureLess resistantMore resistant
Resistance to UV raysLess resistantMore resistant
Difficulty of applicationEasyDifficult
Drying timeFastSlow

Ultimately, the best type of polyurethane for you will depend on your specific needs and preferences. If you are looking for a clear, easy-to-apply finish for indoor use, water-based polyurethane is a good option. If you are looking for a durable finish for outdoor use, oil-based polyurethane is a better choice.

What Can Cause A Coat of Polyurethane To Feel Rough?

There are a few things that can cause a coat of polyurethane to have a  rough surface. These include:

  • Insufficient preparation. If the wood surface is not fully cleaned before you apply the polyurethane finish, the surface will be rough when it is dried due to dust and debris.
  • Applying too much polyurethane. If you apply too much polyurethane, it will not be able to dry properly and will feel rough.
  • Not sanding between coats. It is important to sand between coats of polyurethane to remove any imperfections and to create a smooth surface for the next coat to adhere to.
  • Using the wrong type of polyurethane. There are different types of polyurethane available, and some are better suited for certain applications than others. If you use the wrong type of polyurethane, it may not dry properly or may feel rough.
  • Not allowing the polyurethane to dry completely. Polyurethane needs time to dry properly. If you apply another coat before the previous coat is dry, it will not be able to adhere properly and will feel rough.

If you have a rough coat of polyurethane, there are a few things you can do to fix it. You can lightly sand the surface with 220-grit sandpaper, wipe it down with a tack cloth, and then apply another coat of polyurethane.

You can also try thinning the polyurethane wood finish with mineral spirits before applying it. If the polyurethane is still rough after these steps, you may need to use a different type of polyurethane or apply more coats.

Here are some tips to help you avoid a rough coat of polyurethane:

  • Clean the wood surface thoroughly with a tack cloth or a damp cloth.
  • Apply the polyurethane in thin coats.
  • Sand between coats with a fine-grit sandpaper like a 220-grit.
  • Allow the polyurethane to dry completely between coats.
  • Use the right type of polyurethane for the application.

What Can Cause Air Bubbles To Form In A Coat of Polyurethane?

You may have encountered air bubbles forming in the polyurethane after application, and that’s pretty annoying.  You’ve done everything right in preparing the wood, and now the topcoat has bubbles.

Why, you ask? 

There are a few things that can cause polyurethane bubbles to form in a coat of polyurethane. These include:

  • Shaking the can. Shaking the can of polyurethane introduces air bubbles into the finish, which will appear as small bumps on the surface.
  • Using a dirty brush. If the brush is dirty, it will pick up dirt and dust from the surface, which will cause bubbles to form. A foam brush can be a smart choice when applying poly.
  • Applying the polyurethane too thickly. If you apply the polyurethane too thickly, it will not be able to dry properly and will form bubbles.
  • Working in a humid environment. If you are working in a humid environment, the polyurethane will dry more slowly and will be more likely to form bubbles.
  • Using the wrong type of polyurethane. There are different types of polyurethane available, and some are more prone to bubbling than others. If you are using a polyurethane that is not designed for use on wood, it may be more likely to form bubbles.

If you have bubbles in your polyurethane finish, there are a few things you can do to fix them. You can try to pop the bubbles with a pin or needle, but this may not work if the bubbles are large.

You can also try sanding the entire surface lightly with a fine-grit sandpaper, something like a 220-grit sandpaper, using a sanding block, dry sanding, or even wet sanding, wiping it down with a tack cloth, and then applying another coat of finish.

If the polyurethane bubbles are still visible after these steps, you may need to strip the finish from the entire surface and start over or become a little bit more aggressive in your sanding.

Here are some tips to help you avoid bubbles in your polyurethane finish:

  • Do not shake the can. Instead, stir the polyurethane slowly and gently. This is the most common reason for bubbles forming in polyurethane – be sure to stir (see that little rhyme?).
  • Use a clean brush. Wash the brush thoroughly with soap and water before using it.
  • Apply the polyurethane in thin coats. This will help the polyurethane to dry properly and reduce the chances of bubbles forming.
  • Work in a well-ventilated area. This will help to prevent the polyurethane from drying too quickly and forming bubbles.
  • Use the right type of polyurethane. If you are unsure what type of polyurethane to use, consult the manufacturer’s instructions.

How To Deal With Brush Marks in Your Polyurethane

Choosing The Right Polyurethane

Brush marks are a common problem when applying polyurethane, too. They can be caused by a number of factors, including:

  • Using a dirty brush. A dirty brush will pick up dirt and dust from the surface, which will show up as brush marks in the polyurethane finish.
  • Applying the polyurethane too thickly. If you apply the polyurethane too thickly, it will not be able to dry properly and will leave brush marks.
  • Applying the polyurethane in the wrong direction. Polyurethane should be applied with the grain of the wood. If you apply it against the grain, it will leave brush marks.
  • Not sanding between coats. It is important to sand with a fine-grit sandpaper between coats of polyurethane to remove any imperfections and to create a smooth surface for the next coat to adhere to. If you do not sand between coats, the brush marks from the previous coat will be visible in the next coat.

There are a few things you can do to avoid brush marks in polyurethane:

  • Use a clean brush. Wash the brush thoroughly with soap and water before using it.
  • Apply the polyurethane in thin coats. This will help the polyurethane to dry properly and reduce the chances of brush marks forming.
  • Apply the polyurethane with the grain of the wood.
  • Sand between coats.  A light sanding with a fine-grit sandpaper, like  220-grit should work well for this task. You might even choose to use a 0000 grade steel wool to create a smoother finish.
  • Use a high-quality polyurethane. A good-quality polyurethane will be less likely to leave brush marks.

If you do get brush marks in your polyurethane finish, there are a few things you can do to fix them. You can lightly sand the surface with 220-grit sandpaper, wipe it down with a tack cloth, and then apply another coat of polyurethane.

You can also try thinning the polyurethane with mineral spirits before applying it. If the brush marks are still visible after these steps, you may need to strip the finish and start over.

Here are some additional tips to help you avoid brush marks in polyurethane:

  • Work in a well-ventilated area. This will help to prevent the polyurethane from drying too quickly and forming brush strokes.
  • Apply the polyurethane in a consistent manner. Don’t overload the brush and apply it evenly.
  • Let the polyurethane dry completely between coats.

What Else Can Cause a Coat of Polyurethane to Feel Rough?

Dust nibs.  Yes, dust nibs.  

Dust nibs can cause a polyurethane coating to feel rough. Dust nibs are small bits of dust or debris that get trapped in the polyurethane finish as it dries. They can cause the finish to feel rough and uneven.

Here are some tips to avoid dust nibs:

  • Work in a clean environment. Make sure that the work area is free of dust and debris before you start applying the polyurethane.
  • Use a tack cloth. A tack cloth is a lint-free, clean cloth that is used to remove dust and debris from a surface. Apply the clean cloth to the surface before you start applying the polyurethane.
  • Apply the polyurethane in thin coats. Thin coats will dry more quickly and will be less likely to trap dust nibs.
  • Let the polyurethane dry completely between coats. This will give the dust nibs time to settle out of the finish.

If you do get dust nibs in your polyurethane finish, you can try to remove them by lightly sanding the surface with 220-grit sandpaper. You can also try using a de-nibbing agent, which is a product that is specifically designed to remove dust nibs from polyurethane finishes.

Finally, you could choose a 0000 grade steel wool to smooth the surface.

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Here are some additional tips to help you avoid dust nibs:

  • Use a high-quality polyurethane. A good-quality polyurethane will be less likely to trap dust nibs.
  • Apply the polyurethane with a clean brush. A dirty brush will pick up dust and debris from the surface, which can cause dust nibs.
  • Work in a well-ventilated area. This will help to prevent the polyurethane from drying too quickly and trapping dust nibs.

What Is Grain Raising in WoodWorking?

A raised grain will make a wood surface feel rough, too. Again, it’s a rough finish that we are trying to avoid – 

Grain raising is a phenomenon that occurs when wood fibers swell and raise up from the surface of the wood when it is exposed to moisture. This can happen when water-based finishes such as a water-based poly are applied to wood, or when wood is exposed to humid conditions.

The wood fibers in a piece of wood are arranged in a way that resembles a bundle of straws. When the wood is exposed to moisture, the fibers swell and raise up from the surface. This is because the moisture causes the cell walls in the wood to expand.

Grain raising can cause a number of problems, including:

  • A rough surface. The raised fibers can make the surface of the wood feel rough and uneven.
  • A dull finish. The raised fibers can prevent the finish from adhering properly to the surface of the wood, which can result in a dull finish.
  • Cracking. If the raised fibers are not sanded smooth, they can crack as the wood dries, which can ruin the finish.

There are a few things that can be done to prevent grain raising:

  • Use a water-resistant finish. If you are using a water-based finish, it is important to use a finish that is specifically designed to be used on wood. These finishes will help to prevent the wood fibers from swelling.
  • Apply the finish in thin coats. This will help to prevent the finish from trapping moisture in the wood, which can cause the fibers to swell.
  • Let the finish dry completely between coats. This will give the finish time to soak into the wood and prevent the fibers from swelling.
  • Sand the surface smooth before applying the finish. This will help to remove any loose fibers that could cause the finish to crack. A fine-grit sandpaper will, or should, do the trick.

If grain raising does occur, it can be sanded smooth with a fine-grit sandpaper like a 220-grit or 320-grit sandpaper. However, it is important to sand in the direction of the grain to avoid damaging the wood.

Can You Pre-Treat Wood Before Using a Water-Based Polyurethane?

Shellac

Yes, you can pre-treat wood before using a water-based polyurethane. This is especially important if you are using a softwood, such as pine, as these woods can be more prone to blotching and uneven absorption.

There are a few different products that you can use to pre-treat wood, including:

  • Wood conditioner : A wood conditioner is a clear liquid that is applied to wood to help it absorb the polyurethane evenly. It also helps to prevent the wood grain from raising and creates a smooth surface for the polyurethane to adhere to.
  • Shellac : Shellac is a natural resin that is dissolved in alcohol. It is a good choice for pre-treating wood because it is non-toxic and dries quickly.
  • Mineral spirits : Mineral spirits are a clear liquid that is used to clean and prepare wood surfaces. They can also be used to thin polyurethane, which can make it easier to apply.

If you are using a water-based polyurethane, it is important to use a pre-treatment product that is specifically designed for use with water-based finishes. This will help to prevent the polyurethane from lifting or bubbling.

To pre-treat wood, simply apply the product to the surface of the wood with a natural bristle brush, or a synthetic brush, or clean cloth. Allow the product to dry completely before applying the polyurethane.

Here are some tips for pre-treating wood before using a water-based polyurethane:

  • Use a clean brush or cloth. A dirty brush or cloth will contaminate the pre-treatment product and could cause problems with the polyurethane finish.
  • Apply the pre-treatment product evenly. This will help to ensure that the polyurethane absorbs evenly.
  • Allow the pre-treatment product to dry completely. This is important to prevent the polyurethane from lifting or bubbling.

Video Demo on Sanding A Polyurethane Topcoat

We found a video about repairing a bad polyurethane application – brush marks, bubbles, dust nibs, and even bugs.  

It’s not fatal.  Good wood surface preparation, including pre-treatment, no shaking, sanding between coats, sandpaper or steel wool, etc…follow our suggestions, and that final coat of poly will be just fine.

Last update on 2024-04-13 / Affiliate links / Images from Amazon Product Advertising API

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