Last Updated on April 7, 2023 by Ernest Godia
Applying polyurethane to hardwood floors and other wood projects may appear straightforward for experts and experienced DIYers. However, unless you know what happens if you don’t sand between coats of polyurethane, you can easily skip sanding where you shouldn’t.
Such a mistake can be daunting. Fortunately, this article has everything you need to know about sanding between coats of polyurethane. This includes the following:
- Why you should sand between coats
- What happens if you don’t sand between coats
- How to sand between coats of polyurethane
- Choosing the correct sandpaper grit for polyurethane coats
- The best options to consider when you don’t want to sand between coats of poly
Do You Have To Sand Between Coats Of Polyurethane?
Sanding between coats of polyurethane is necessary with some polyurethanes but optional with others. Nonetheless, it helps remove imperfections such as dust nibs, brush marks, and bubbles created while applying the sealer.
Ideally, water-based polyurethanes do not usually require sanding between coats to adhere properly. On the other hand, oil-based polyurethanes require sanding between coats for proper adhesion.
The exercise influences the adhesion of the subsequent coats of oil-based and high-gloss polyurethanes.
Still, you should always scuff sand between coats regardless of the polyurethane type for aesthetic reasons. Doing so will help you remove imperfections and achieve a clean and smooth finish.
What happens if you don’t sand between coats of polyurethane
Most types of polyurethane, especially oil-based or glossy types, will not adhere or bond properly without sanding between coats. This means such finishes will likely start peeling or chipping off once dry if you don’t sand between coats.
Other types of polyurethane, mainly water-based or low-sheen types, will remain intact without chipping or peeling despite not sanding between coats.
However, you may end up with a bumpy or untidy finish if you don’t sand since sanding helps remove dust nibs and bubbles trapped between coats of poly. This may happen for any polyurethane—water- or oil-based.
Since failing to sand between coats of a polyurethane type that requires sanding results in adhesion problems, you may have to redo the work sooner. The sealant will form layers that dry separately, flaking and peeling with exposure to temperature changes.
The finish will end up being less durable, forcing you to refinish the wood within just a few months.
Why do some types of polyurethane need sanding between coats and not others?
Some types of polyurethane require sanding between coats, while others do not because they behave differently when you apply multiple coats.
Fresh water-based polyurethane reacts to dried coats of water-based polyurethane, merging with them. Therefore, a water-based poly will adhere and bond to a previous coat during recoating without problems, even if you do not sand it.
On the other hand, oil or solvent-based poly dries to form separate layers when multiple coats are applied. So you must sand the dried coat before applying a fresh one for them to bond and avoid peeling off when dry.
Nonetheless, you should always consider sanding between coats of polyurethane, even when the manufacturer indicates that the product does not require sanding.
Sanding serves two sound purposes, namely:
- To ensure proper adhesion between the coats
- Remove any surface imperfections, including brush marks, bubbles, and dust.
While some polyurethane manufacturers recommend not sanding, they usually base their recommendation on adhesion and not aesthetics.
Notice that even polyurethane formulations that adhere to previous coats without sanding can still trap dust particles and bubbles.
Without sanding, these imperfections will show through the finish. You can avoid that problem by sanding between coats of polyurethane, regardless of what the manufacturer recommends.
Details of what happens if you don’t sand between coats of polyurethane
You now know what happens when you don’t sand between coats of polyurethane. Here is a detailed breakdown of what can go wrong when you skip sanding between coats of polyurethane.
- Dust particles and bubbles may be trapped between the coats
Polyurethane is a liquid plastic, so it can easily trap dust nibs when applying it. The larger the surface, the more likely it will trap dust particles. So if you are finishing a hardwood floor, it will almost certainly trap some dust along the way.
Bubbles equally tend to form on polyurethane surfaces when applying it, regardless of the method used. If you are just starting out with applying polyurethane, the chances of bubbles forming are almost 100 percent.
While these bubbles naturally disappear within minutes, they can leave the surface feeling rough.
Sanding helps remove these blemishes, including dust particles and bubbles, making the finished wood clean and smooth.
- The subsequent coats of poly may not adhere properly
As already pointed out, some polyurethane formulations form layers when they dry. This is especially true for oil-based poly.
Such formulations create slippery layers that prevent chemical bonding. Therefore, the finish will likely chip and peel if you don’t sand between coats.
Sanding creates a coarse texture that gives the subsequent coat something to grip on. Otherwise, the individual layers will sit one on top of the other instead of fusing together.
This causes them to separate over time as they dry. Eventually, the changes in temperature and humidity cause the upper coats to peel off.
- You will be unable to remove brush marks.
The other culprit behind polyurethane surface imperfection is brush marks. Some polyurethanes are formulated as self-leveling, meaning they naturally lose the brush marks during drying without additional input from you.
However, for standard polys that do not self-level, you must knock down the brush marks with sandpaper to create a smooth finish.
Without doing so, unsightly marks will show through the final coat of the sealant. Such results will look anything but professional, irrespective of your skill level.
How to sand between coats of polyurethane
Unlike sanding wood, polyurethane or any finish requires some care and the right choice of tools and equipment. Improper sanding can mean removing the entire finish and undoing all the work you have done.
You don’t want that. So here are the essential tips and guidelines when sanding between coats of polyurethane.
- Use very fine grit sandpaper. A 220 or higher grit should do an excellent job of scuffing the surface without eating into the finish.
- Sand in the direction of the grain. This approach helps ensure you do not leave scratches in the polyurethane finish.
- Allow the polyurethane coat to dry thoroughly before sanding it. You should see white powder forming while sanding if the coat is thoroughly dry unless the urethane is lousy.
- Observe the drying time for your specific polyurethane before attempting to sand. Water-based poly should be dry in less time than oil-based polyurethane under the same weather conditions. Check the product label for the recommended recoat times.
- Use a palm sander or sanding block for a flat surface or a sanding sponge for a contoured wood surface. Do not use a random orbital sander, as its powerful vibration means it will sand right through the finish, taking you back to square one.
- Keep the pressure light while sanding to avoid removing the coat you already applied. The idea is to flatten the surface, not sand through the finish.
- Use an electric finishing sander for large projects to be quick and efficient, but keep the pressure very light and even along the grain. It is crucial to keep the sandpaper moving and not lingering in the same spot.
- After sanding the entire surface, use a vacuum with a brush attachment to suck up the white powder formed during sanding. Follow it up with a painter’s tack cloth to eliminate any remaining traces of sanding dust.
Here is a video illustration of the correct way of sanding between coats of polyurethane.
What grit sandpaper to use between coats of polyurethane?
The best grit sandpaper for sanding between coats of polyurethane is 220 and higher. So you can use 320- or 400-grit sandpaper between coats of polyurethane with great success since the aim is to scuff up the surface and knock down the imperfections, not remove the finish.
Consider sanding by hand, especially for smaller projects. However, if the project is big, you can use an electric finishing sander and keep the pressure light.
Do not use powerful sanding tools like an orbital sander. The vibrations will likely dig through the finish, undoing your work.
How many coats of polyurethane should you apply?
The number of coats of polyurethane applied depends on whether it is oil-based or water-based.
For a water-based finish, you can apply as many coats as four to six, especially for heavy-use items such as hardwood floors.
On the other hand, oil-based polyurethane requires two to three coats. Applying more than three coats may result in adhesion problems.
Oil-based poly contains a high concentration of solids, making them thicker. For this reason, you should achieve the required durability and protection with fewer coats than water-based polys, which have fewer solids, and hence require more coats to achieve the desired protection.
Read our detailed comparison of water-based and oil-based polyurethane for more details.
Can I apply too many coats of polyurethane?
Yes, you can apply too many coats of polyurethane, especially if the product is oil-based. Oil-based poly requires a maximum of three coats. Any additional coats after the third one may not adhere properly and could peel off.
While you could potentially apply an infinite number of water-based polyurethane, too many coats may still cause problems such as cracking due to the overloading of the finish.
So whether you sand between the coats or not, ensure you keep the number of polyurethane coats to the acceptable limit.
How long should I let polyurethane dry before sanding?
Always wait for the drying time recommended on the label of your specific polyurethane or longer if the temperature is unideal before sanding it. Sanding before the surface is thoroughly dry is not recommended, as the sealant will be gammy and unlikely to sand properly.
Water-based polyurethane should be ready for sanding and recoating in 6 to 12 hours. Oil-based polyurethane takes longer to dry, so it will require at least 24 hours of drying time before you can sand it and recoat.
There is an exception to this rule, with brands like Minwax having fast-drying variants that can be recoated sooner.
For example, Minwax fast-drying polyurethane is an oil-based clear sealer that can be ready for sanding and recoating in 4 to 6 hours.
The brand’s fast-drying water-based polyurethane variants allow for sanding and recoating in as little as 2 hours.
However, we do not recommend using fast-drying polyurethane variants if you are a beginner. Instead, they guarantee the best results in the hands of experienced users.
What to do if you don’t want to sand between coats of polyurethane
While sanding between coats is recommended for all polyurethane types, you can skip it if doing so is absolutely necessary.
However, ensure you choose one of these available options that allow you to safely and successfully seal your wood without sanding between coats.
- Choose products that don’t require sanding between coats
Some polyurethanes do not require sanding. If you must skip sanding, you can go for these types, often water-based formulations, preferably self-leveling.
Once you have a water-based sealant or special oil-based poly that is self-leveling, it leaves you with only dust and bubbles to worry about.
With good skill and careful application, you should end up with decent results without sanding between coats.
- Use a single-coat polyurethane.
Alternative to products that don’t require sanding is those that need only one coat. Usually, you don’t sand the final coat of polyurethane in any application. So working with only one coat naturally eliminates sanding.
Even though such products are usually oil-based, they tend to dry fairly quickly. This can be an excellent way to speed up your project completion and save yourself some elbow grease.
Notice, however, that one-coat polyurethanes are pretty thick and can be problematic to work with. So it may not be the ideal choice for a beginner.
- Do not use a glossy finish.
High-gloss oil-based finishes show imperfections more visibly than satin and matte finishes.
For polyurethane, the glossier it is, the more likely it is to form layers between coats instead of fusing into a single coat of wood finish.
So, consider matte and satin options if you don’t intend to go all the way with sanding between coats.
Can you put a second coat of polyurethane without sanding?
Yes, you can add a second coat of polyurethane without sanding if the product manufacturer specifies it does not require sanding between coats. However, dust particles, bubbles, and brush marks can show through the final coat, keeping the finish from being clean and smooth.
Can you apply polyurethane over old polyurethane without sanding?
Yes, you can apply water-based polyurethane over old water-based polyurethane without sanding. However, you must sand the old layer of polyurethane if you are using an oil-based poly.
How long does it take for polyurethane to dry between coats?
Water-based polyurethane takes about 6 to 12 hours to dry between coats, while oil-based poly takes at least 24 hours to dry. In bad weather, oil-based polyurethanes can take up to 48 hours to dry between coats.
Conclusion on sanding between coats of polyurethane
Sanding between coats of polyurethane is quick and simple as it requires only a few light passes with a fine-grit sandpaper. But it can make the difference between an OK finish and a perfect one.
Sanding between coats not only improves adhesion but also helps smoothen the surface by removing imperfections from bubbles, dust, and brush marks. If you want a professional-looking finish, sanding between coats of polyurethane is essential.
But if you must skip sanding for any reason, consider one of the alternatives provided here for the best results.
We hope this helps you out. Let us know your thoughts in the comments.