Polyurethane coat application might sound like a piece of cake until you can’t decide whether to sand between the coats or not. But do you have to sand between coats of polyurethane? Why is it necessary? And what happens if you don’t sand between coats of polyurethane?
For some types of polyurethane finishes, nothing happens if you don’t sand between coats. However, the most common problems regarding polyurethane application involve light sanding, regardless of polyurethane type. Therefore, it is recommended to sand between polyurethane coats to eliminate the problems and achieve a perfect finish. Let’s dive into more detailed explanations to answer these questions and more.
Do You have to Sand Between Coats of Polyurethane?
Polyurethane application benefits your project with protection, aesthetics, and improved durability. To achieve all these, you must properly apply polyurethane coats, made possible by sanding between polyurethane coats. The primary reason for sanding between coats of polyurethane is to remove bubbles, dust nibs, and other imperfections from the surface.
Applying the next coat after 24 hours on a sanded surface will also help you create a stronger adhesion with the previous coat. However, fast-drying and most water-based polyurethane products will remain glossy even after sanding, which won’t help with adhesion.
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Choosing to sand between polyurethane coats will also depend on the manufacturer and the type of polyurethane you use. For instance, some water-based polyurethane products do not require sanding between coats, provided you observe specific recoating times, but most oil-based polyurethanes require sanding between coats. However, besides the technical aspects, sanding between polyurethane coats is essential for a perfect finish.
Reasons for Sanding Between Coats of Polyurethane
There are two reasons why you should sand between polyurethane coats;
First, when recoating, you must ensure maximal adhesion between the coats. If the subsequent coat does not integrate and stick to the preceding coat, you’ll not get the lifespan you anticipate from the surface finish. With time, the coatings will begin to peel off with contraction, expansion, and use, compromising the surface protection and ruining your project. Therefore, sanding the glossy moisture-resistant surface will slightly roughen it, allowing the subsequent coat to stick.
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Additionally, sanding between coats removes dust, brush marks, bubbles, and other surface imperfections on the preceding coat. Therefore, light sanding the intermediate coat smooths the surface in preparation for the subsequent coat and prevents sealing in defects that detract from the final finish.
What Happens If You Don’t Sand Between Polyurethane
Here are a few things that can go wrong if you fail to sand between polyurethane coats;
1. Bubbles and Dust Can Get Trapped in Lower Coats
When applying polyurethane, there will always be bubbles, but most disappear within minutes. The remaining bubbles can be easily sanded. On the other hand, dust particles can appear if the wood grain isn’t cleaned properly before applying the polyurethane finish or if the sanding dust is blown onto the wood surfaces.
Weathering the wood’s surface again will eliminate dust nibs and bubbles. If you don’t remove the dust nibs, the finished project will look dirty and stay that way, no matter how frequently you try to clean it. Bubbles, too, will make your work have a lower-quality appearance.
2. The Subsequent Polyurethane Coats will not Adhere Properly
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Polyurethane is slippery, and its formula prevents the bonding of chemical solvents. If you don’t sand between the coats, each will dry independently, creating layers. When the surface is exposed to humidity and temperature changes, the layers will start to delaminate and peel off, making the surface rough and unpleasant.
Besides the adhesion, the main advantage of sanding polyurethane coats is that they allow you to inspect your work and check for flaws.
3. You Will not be Able to Remove Brush Marks
Brush marks are another unsightly error that can happen when applying polyurethane. Although some suggest using a high-quality foam brush, this can be effective if you want only one coat, and it will only assist in reducing brush marks and not other surface contaminants and defects.
Polyurethane is a thick, heavy liquid that takes time to level out fully. Bubbles and brush marks may disappear as it levels but not fully. The initial layers of polyurethane coats may show brush strokes from certain products, but the final layer usually conceals these imperfections.
When you notice brush marks on the coated surface, sand lightly over them and reapply another coat of polyurethane. Applying a subsequent coat of polyurethane will make the brush marks disappear, but only after sanding. Also, cleaning the application brush can help prevent uneven brush marks.
Best Options If You Want to Skip Sanding Between Polyurethane Coatings
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If you are impatient with your project and can’t go through the sanding process, here are some options you can consider.
1. Use Polyurethane Products that Don’t Require Sanding
Some polyurethane products don’t require sanding. These are mostly water-based polyurethane. Also, you can find oil-based products that don’t require sanding but give decent results.
2. Don’t Use a Gloss Finish
Do not use a high gloss finish for oil-based polyurethanes. Instead, use semi-gloss. The gloss doesn’t have curing agents like satin or semi-gloss polyurethane formula finishes. Consequently, defects show up more, and subsequent coats do not adhere well without sanding.
On the other hand, satin can be applied, with minimal repercussions, without sanding.
3. Use One Coat of Polyurethane
Another option is to use polyurethane products that require only one coat, no sanding, no additional coat, and no waiting. Also, such products normally dry faster, despite being oil-based. However, one-coat poly is usually very thick, and they don’t level out well. As a result, they tend to show brush strokes because they dry quickly. Therefore, they are not recommended for beginners.
But, if you are a professional, who doesn’t mind a bit of experimentation or working on a small area, you can try working with one-coat poly and skip the sanding process.
How to Sand Between Coats of Polyurethane
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Ensure the first coat of polyurethane has dried to the point the surface can’t be damaged when you begin sanding. For oil-based polyurethane, you can wait four hours or longer if the temperature is cold. You can get away with two hours for water-based polyurethane, but you shouldn’t rush it.
Use 220-grit sandpaper to sand polyurethane and gradually increase the grit as you add more coats. The sanding aims to remove imperfections from the polyurethane and flatten the surface but not to remove the coat you’ve just applied. Apply light pressure and keep the sandpaper moving, don’t remain in one spot.
After sanding the whole surface, ensure you vacuum the surface and the surrounding area to remove all the sanding dust. Then use a tack cloth to remove any remaining traces of dirt and dust. Once the surface is scrupulously clean, you can apply the next thin coat.
How Long Should You Wait for Polyurethane to Dry Between Coats
Waiting for the coat to dry before you sand and apply the next coat is tedious, especially after waiting for the stain to dry before applying polyurethane. But is safer than ruining your whole project. Therefore, it is important to know how long your polyurethane coat will take before it is dry.
Waiting time majorly depends on the type of polyurethane and the manufacturer. Oil-based polyurethane dries slower than water-based polyurethane. Oil-based poly takes 12 to 24 hours to dry, and most companies recommend waiting the whole day. On the other hand, water-based polyurethane can dry between 6 to 12 hours.
However, some brands have fast-drying variants. For example, you can find a fast-drying oil-based poly that can take between 4 to 6 hours and water-based poly that can be recoated in 2 hours. But don’t begin sanding too early lest you risk dragging a still-wet surface, ruining it.
Most importantly, you should read the manufacturer’s instructions because each product differs. However, the instructions are usually based on thin coats and in average conditions such as good ventilation, relative humidity of 50%, and a temperature of 77 degrees Fahrenheit. So if you work in higher humidity or colder temperature environments, the drying time will take longer.
Frequently Asked Questions
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1. Should I sand the final coat of polyurethane?
If you have properly sanded the lower coats, the final one, normally the third coat, should come out smooth and look great. Therefore, sanding it will scratch the surface and leave a cloudy appearance. So only sand between the poly coats and let the final coat dry without sanding to achieve a smooth surface finish. You can use a tack cloth to remove any remaining traces of dirt and dust.
2. What grit sandpaper should I use to sand between polyurethane coats?
Sanding between polyurethane coats aims to help them adhere properly and not scratch the surfaces, as is the case for removing pencil marks and other stubborn marks. Therefore, lightly sanding the coats will improve the adhesion without affecting the surface appearance. For most applications, 320 sandpaper grit will suffice. However, you can use finer sandpaper grits like 400 for flawless results.
3. How many coats of polyurethane should I apply?
Most companies recommend applying at least three coats of polyurethane for the best finish. However, if your project is not to be used often, then two coats will work well, but for heavily trafficked areas like floors, you should apply at least three polyurethane coats.
Additionally, use three to four coats if you use oil-based polyurethane on a floor. But if you use a water-based poly, you can use five to six coats. Water-based polyurethane is normally thin; you should apply it 1 to 2 times more than an oil-based product.
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Despite all the impressive products on the market, nothing beats the results you will get when you sand between polyurethane coats. It might take time and a bit of money, but not sanding a polyurethane that needs sanding will significantly shorten the lifespan of the surface finish. Similarly, sanding between polyurethane that doesn’t need it will not hurt the finish.
Additionally, sanding between coats of polyurethane is the only way to quickly rectify errors and prevent you from starting all over. So, lightly sand between polyurethane coats to get a smooth and even finish. Also, remember that there are no shortcuts to perfection in polyurethane application.
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