I heard a troubling stat the other day that 90 percent of people that use polyurethane apply it wrong. 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 sanding.
In this post, we are going to look at:
- The role of sanding when applying polyurethane
- Choosing the right grit
- 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 poly finish will eventually peel off or become deformed.
With so many different types and manufacturers out there, 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, then you might not have a choice but to go for the easiest one to apply.
But be warned – there are no shortcuts to perfection.
Why do Some Types of Polyurethane Need Sanding Between Coats and not Others?
Sanding is really time-consuming work. Sure, the most challenging part is getting the wood 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 just 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 finishes are the ones that don’t need sanding between coats; all you have to do is keep the floor clean with a tack cloth. There aren’t many of these in the market, so research is sparse and positive reviews are even rarer.
Second Coat of Polyurethane without Sanding? 3 Concerns.
When preparing to write this article, I wanted to find out what already exists on the internet, and what I saw was very disturbing. A lot of websites said “nothing serious” will happen if you don’t sand between 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, so here are some of the things that can go wrong when you don’t sand between coats of polyurethane:
1. The Subsequent Coats Won’t Adhere Properly
Polyurethane, especially oil-based poly, is a unique product. It’s not like paint or lacquer that don’t need to be sanded between coats. You see, poly is kind of slippery and has a different makeup that prevents chemical bonding.
When the paint dries, you can apply another coat, and it sticks together. However, what happens with 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. The question is, do you really want to risk your hard work peeling off after a few days or weeks?
2. Dust and Bubbles Can Get Trapped in Your Lower Coats
One of the reasons we sand poly is to remove dust nibs 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 nibs, 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 floor properly before applying your finish or dust blown into the room when you don’t dispose off the sanding dust properly.
But quite often, they just appear.
In either case, sanding the 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 will remain that way no matter how many times you try to clean it.
As we can see from the picture, 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.
3. You Won’t be Able to Remove Brush Marks
Another unsightly and painful error when applying polyurethane is leaving brush marks. Then, again, this has little to do with you as a user and more to do with the product.
Polyurethane is a thick, heavy liquid, and it takes time to fully set or level out. 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 coats of polyurethane. By the third coat, these imperfections are usually gone, although not by magic.
When you notice any brush marks, sand over it and reapply a 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 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 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 coats besides the adhesion is that it gives you the chance to inspect your work and check for flaws.
When you are in a hurry to finish, and you apply the next coat, you may not take the time to intentionally look for mistakes. The consequence of rushing is that you risk compromising all the work you have done.
What Type of Sandpaper Should I use Between Coats?
The aim of sanding between coats of polyurethane is to help it adhere better and not distort the previous one, so you just need to sand lightly. For most projects, 220 grit sandpaper will suffice. However, you may also use finer grits like 320 or 400, and the end result will still be pristine.
Regarding 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 coats, then electric sanders will come in handy.
How Many Coats of Polyurethane Should I Apply?
Is 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 three coats.
Oil-based finishes, in particular, often require three coats. Remember that the first coat will be buffed, so it will lose some of its luster. The same happens to the second coat. Usually, the polyurethane will be level by the third 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 coats will rarely or probably never ruin your work. However, you have to sand between 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 third 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 then apply the next one. 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 polyurethanes take a lot longer to dry than water-based. Oil polys 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 polys 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 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 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, brush 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. 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 gloss finish. Gloss does not have flatting agents like semi-gloss or satin. As a result, flaws show up more, and subsequent coats don’t adhere as 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 second 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 marks.
Personally, I don’t use it, and I also don’t recommend it for beginners.
However, if you`re a professional, and you are 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.
To Sand or not to Sand?
Despite all the fancy products on the market right now, nothing beats the results you get when you sand between coats. Yes, it takes time and a bit of money, but it yields a smoother, better quality finish.
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. For those yet to do either, leave a comment below to let me know how it goes.