I heard a troubling stat the other day that 90 percent of people that use polyurethane apply it wrong. Should I sand between coats of polyurethane? Whether this is fact or factoid, I do not know.
What I do know, however, is that many of the common problems regarding polyurethane have to do with light sanding.
In this post, we are going to look at:
- What Happens if you Don’t 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
Lest I drive you nuts, I’m going to kill the suspense as we dive into more detailed explanations.
What Happens if You Don’t Sand Between Coats of Polyurethane?
For some types of poly, nothing happens if you don’t sand between coats of polyurethane. However, most polyurethane will not adhere properly if you skip sanding dust nibs and brush marks on the finish. The polyurethane finish will eventually peel off or become deformed.
With so many different types and manufacturers, it can be hard to know what to choose. But, of course, you should always choose the product that suits your project. If you are pressed for time, for example, you might not have a choice but to go for the easiest one to apply.
But be warned – there are no shortcuts to perfection during polyurethane application.
Why do Some Types of Polyurethane Need Sanding Between Coats and not Others?
Thorough sanding between coats of polyurethane is really time-consuming work. Sure, the most challenging part is getting the wooden surfaces ready. However, even after applying the first coat of polyurethane, it still takes a while.
What is worse is that you’ll probably have to use manual sanding methods instead of the machines unless you are a professional, which delays you even further.
Manufacturers know that you don’t have a lot of time, and they want to help you move on as quickly as possible. That is why we now have way more fast-drying polyurethanes than we did when I first started woodworking many years ago.
The next step up from fast-drying polyurethane finishes are the ones that don’t need wet sanding between coats; all you have to do is keep the wood floor clean with a tack cloth.
Unfortunately, there aren’t many of these in the market, so research is sparse and positive reviews are rare.
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’m going to keep my cool and tell it as it is (deep breaths).
Okay, in regards to this question, here are some of the things that can go wrong if you fail to lightly sand between coats of polyurethane.
What Happens If You Don’t Sand Between Coats of Polyurethane
1. Dust and Bubbles Can Get Trapped in Your Lower Coats
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 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 get rid of the dust nibs, your finished project will look dirty and remain that way no matter how many times you try to clean it.
Needless to say, bubbles make your work look diseased and just 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. The 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 slices of cheese 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 really want to risk your hard work peeling off after a few days or weeks?
3. You Won’t be Able to Remove Brush Marks
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 gives you the chance to inspect your work and check for flaws.
What Grit Sandpaper between Polyurethane Coats?
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.
How Many Coats of Polyurethane Should I Apply?
Is only one coat of polyurethane enough? Absolutely not!
So, how many coats of polyurethane should you add? For the best finish, most manufacturers recommend applying at least three coats.
If the piece of furniture is not going to be used often, then two coats will suffice. For heavily trafficked areas like floors, however, you should apply polyurethane coats not more than three.
Oil-based polyurethane finishes, in particular, often require three coats. Remember that the first coat will be buffed, so it will lose some of its lusters.
The same happens to the second and last coat. Usually, the polyurethane will be level by the next coat, all imperfections will be gone, and your work will look outstanding.
Can I Apply too Many Coats of Polyurethane?
Yes, and no. Too many polyurethane coats will rarely or probably never ruin your work. However, you have to sand more coats, so you will spend several weeks on the project, it will cost a lot, and there won’t be any extra protection for your wood.
Typically, the next coat is enough to protect your project for years to come. Do you really need polyurethane to last any longer than that? I don’t think so.
How Long Should I wait Before Each Coat of Polyurethane?
The short and annoying answer is that you have to wait until every coat is dry before you sand and apply polyurethane. But of course, what you want to know is how long it will take before it’s dry.
Well, this depends on the type of polyurethane and the manufacturer.
Oil-based polyurethane dries more slowly than water-based polyurethane. Oil-based poly can take anywhere from 12-24 hours, and most manufacturers recommend you wait the whole day. Water-based poly can be dry between 6-12 hours.
However, some brands have fast-drying variants. For example, Minwax has a fast-drying, oil-based polyurethane that can be recoated in 4-6 hours, and some of their water-based polyurethane can be recoated in 2 hours.
Other brands also have similar products for both water and oil. In fact, some even tout that you can apply subsequent multiple coats without waiting at all.
As promising as that sounds, I will caution you against using these products if you are a beginner. Yes, waiting for the coat to dry is long and tedious, but at least it is safer than having to strip your entire hardwood floor and start all over.
Fast-drying products don’t give you time to correct any mistakes and believe me; there will be plenty of mistakes when you first get started.
You don’t want a situation in which you finish everything, then realize you have bubbles, puddles, runs, pencil marks, streaks, dust nibs, lint, or any other common issue and have to scrape off the entire thing.
That would be extremely careful. Also, bear in mind that these products are more expensive, so if you mess up, you’ve also wasted a lot of money. The slow approach may not be appealing, but it is safe.
What Are My Best Options If I Don’t want to Sand Between Coats?
If you really can’t afford to be patient on this project, here are three options to consider.
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.
I am sure 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.
Satin, on the other hand, can be applied, with minimal repercussions, without sanding. However, don’t take this as a glowing recommendation from me; this is just me sharing your options with you.
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.
Can I recoat polyurethane without sanding?
No, you cannot recoat a polyurethane finished wood without sanding it. To begin, use a 120-150 grit sandpaper or no.2 steel wool to sand the surface lightly. Remove the dust from the wood and clean it. Wipe it with a tack cloth to remove any remaining dust or debris. Reapply polyurethane along the wood grain. Here are some sandpaper alternatives.
Can you sand polyurethane?
Yes, you can sand in between coats of polyurethane 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.
Why sand between coats of polyurethane
The primary reason why you should sand in between coats is to smooth out dust nibs and remove other 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. For fast-drying polyurethane and most water-based poly, it will still remain glossy even after sanding, so it won’t necessarily help with adhesion. In conclusion, it’s necessary to sand polyurethane between coats for a perfect finish.
How to sand between coats of polyurethane
Lightly sand between coats of poly using a 320 grit sandpaper wrapped around a flat block for the first and second coat. The first layer needs to be sanded the most. After the third coat, sand with 400-grit sandpaper, then 500-grit sandpaper. To remove scratches easily, alternate directions with each abrasive.
You may want to watch this tutorial to better understand the sanding process;
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.
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.
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 right now, 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 the work you have done.