We like polyurethane. We use polyurethane a lot on our projects. It provides a durable and hard protective sheet of plastic on the wood and shields it from water spills, can take a reasonable amount of heat, and its finish will last for years.
When applied correctly, it gives a very professional appearance to any project – in the kitchen, throughout the house, and anywhere it is used. Furniture benefits from a polyurethane finish, as do floors, doors, and trim. As long as you understand what you’re working with and how best to apply it, polyurethane will take good care of the projects you worked so hard to build.
The whats, the hows, the whens, and the how many coats all follow.
In This Article
- What Is Polyurethane?
- Polyurethane Used As A Wood Finish
- Types of Polyurethane
- Polyurethane Application
- Polyurethane Application: Number of Coats To Use
- Oil-Based Polyurethane
- Water-Based Polyurethane
- Polyurethane Applications by Project Type
- General Rules To Follow in Applying Polyurethane
What Is Polyurethane?
We turn to it as a topcoat, whether we’ve stained or painted our project, because it forms a film, a plastic sheet, on our project to protect it from water and heat, and we know it will continue protecting it for many years if we have applied it properly and well.
But what is polyurethane? How does it do what it does? Let’s examine its constitution for the answers and the two main substances that comprise its makeup.
Polymers. According to dictionaries, polymers are ”natural or synthetic substances composed of very large molecules.” This may not be as helpful to your understanding as is giving some examples of polymers: synthetic polymers include nylon, polyethylene, epoxy, and Teflon (as in cooking utensil coatings); and natural polymers include silk, wool, proteins, and even DNA.
When we finish our woodworking project with polyurethane, we are not coating it with Teflon, silk, or wool, or even proteins. It is far easier to simply say we are coating our woodworking project in a plastic of sorts.
Clothing, plastic bags, fiberglass, and even silicone are further examples of polymers or polymer-coated items. It is interesting to note that silicone is used to make artificial heart valves, and we know a fellow walking around with one in his chest. He’s doing well as a result of that silicone valve.
Urethanes. As unfathomable as the dictionary definition of what a polymer is, that of urethanes is even more so. It is easiest for our purposes today to say that urethanes, while not exactly in the family of plastics, are more in the family of rubber.
It’s a “tweener,” somewhere between a plastic and a rubber. The polymerization process that takes place in the joining of these two substances is what gives polyurethane its protective properties when a coat of it is applied to wood.
The combination of polymers (poly) and urethanes produces polyurethane.
A Little History of Polyurethane
Polyurethane was originally made in 1937 by Otto Bayer at IG Farben in Germany. It differed from other polymers that were covered by patents, and its uses initially included a coating for airplanes during World War II.
As more uses for polyurethane were found and the composition was adapted to those other uses, it became more ubiquitous in industry. Poly foams were developed, and harder forms of poly became available for use in the making of furniture and clothing (as noted earlier).
The chemical processes and formulas for the ever-growing list of uses to which poly was being put are not necessary to expand upon here for our purposes. Eventually, its utility as a protective coating for wood which flowed out of its use as a coating for airplanes during the war became apparent.
Enter woodworking as a growing market for poly.
That leads us to today when we would expect most of those reading this piece to have worked with poly at some point in their woodworking career.
Polyurethane Used As A Wood Finish
Polyurethane as a wood finish offers many advantages for the protection of your projects. It is stable, safe, heat resistant, and durable, offering good protection for the furniture, trim, doors, floors, cabinets, and countertops you may have used it on.
In its liquid form, poly is applied to wood and dries to a hard and durable plastic sheet that will protect the wood from water and liquid spills of all kinds. We know that water is the enemy of wood, and having an impervious plastic surface on wood keeps that enemy at bay.
It is referred to as a film finish, as it does not penetrate wood; rather, it forms that plastic coating on the surface without being absorbed like a stain would be, or a wood oil. It can be applied over stain or paint and provides the same degree of protection for each.
Types of Polyurethane
There are two types of polyurethane for wood finishes, and each has its advantage depending on the project, the wood, and the use to which the project will be put. Most woodworkers we know have settled on one or the other as their preference, mostly out of habit, familiarity, and the types of projects they undertake.
The first type of polyurethane is water-based, and we’d guess it is the one more commonly chosen by home woodworking hobbyists. It uses water as a medium rather than solvents to carry the poly solids that give it protective qualities.
It dries quickly, and much more so than the second type of polyurethane wood finish, to the protective coating desired for wood, but in virtually all instances, will require more than a single coat to provide the degree of protection wanted.
We’ll talk more about the number of coats for each type of polyurethane in a moment.
While this might make the use of a water-based poly more expensive (more coats = more cost), advancements in the formulas for water-based polyurethanes over the last decade have made it more effective, with fewer coats necessary.
However, more coats make it more effective in resisting water, mold, and fungus.
This type of polyurethane is also a film wood finish that dries hard and provides the same degree of protection to wood as water-based polyurethane. Oil-based polys are durable and have been considered more durable than water-based polys until the advancements in formulas have put them on mostly equal footing in terms of durability.
However, woodworkers who have worked with oil-based polys continue to consider it more resistant to scratches and abrasions. For this reason, it is often the preferred polyurethane for wood flooring finish.
It does require more attention to personal safety when using oil-based polyurethane. Its VOCs, volatile organic compounds, are toxic until it has fully dried (24 – 48 hours) and cured (30 days). Overexposure to them can be very harmful and can lead to respiratory problems and lung damage.
A respirator should always be worn when working with oil-based poly.
Rooms should be well-ventilated when using oil-based poly, and even with a respirator, it is still possible for eyes, nose, and throat to be irritated. Extra care is necessary with them.
Many woodworkers find oil-based polys to be too inconvenient not only because of the VOCs and the problems associated with overexposure but also because it takes so much longer to dry and cure, they tend to darken over time, and are more susceptible to denting than is the water-based type of polyurethane.
Polyurethane has become popular enough to have replaced varnish and shellac for many home woodworking enthusiasts. It can be applied in several ways, with the appropriate personal protective equipment mentioned earlier for the oil-based type.
Brush. Initially, polyurethane had to be brushed onto the wood’s surface, although advances in its chemical composition and manufacture make other methods available. It can be brushed on like paint.
However, brushing poly on can be problematic in that bubbles can form that would require sanding between each coat applied. More work is required, and thus time, when brushing poly on. A foam brush is a better choice than either a synthetic or natural brush – it won’t leave brush strokes to clean up.
Additionally, poly is not necessarily self-leveling, as are other finishes and paints. Brush marks will not smooth out on their own, again requiring sanding between coats.
Rag. A rag application can eliminate those troublesome brush marks. This more hands-on approach will be smoother, less likely to create bubbles in the poly, and with no brush marks, can eliminate the need to sand between coats. There are even polyurethane brands that are formulated specifically for rub-on application.
Spray gun. Yes, polyurethane can be sprayed on wood. We like spray guns. They give us greater control over the evenness of the application; the application will be smoother and more even; there will be less chance for bubbles to form, and no brush marks.
We also believe it gives a much more professional appearance to the finished project.
Should You Thin Polyurethane for Spray Application?
While not absolutely necessary, we believe polyurethane should be thinned for spray application. It will give you much better control over the spray, thus ensuring a more even coat.
You’ll experience fewer textural bumps and less chance for streaking. Thinned poly will dry faster, too. You can avoid a blotchy surface and minimize tackiness. When multiple coats are to be applied, and with thinned poly, you will want more coats than if brushed or wiped on; the task of finishing your project will still be faster, easier, and more convenient.
- For thinning oil-based polyurethane: you will need mineral spirits (a 100% petroleum distillate with no additives); or, lacquer thinner (a cellulose thinner that is a mixture of solvents that dissolve resins and plastics), or naphtha (another petroleum distillate used to thin oil-based substances).
The formula would be 4 parts poly to 1 part thinner of choice. Use a glass vessel to mix so you can see the viscosity as you add the thinner, and test periodically on scrap wood for bubbles or imperfection and to see how it sprays for you.
Again, remember the safety precautions we recommend, and take them seriously even with this testing. The potential for harm is real.
- For thinning water-based polyurethane: water, of course, is one option, and denatured alcohol is the other.
Follow the same process as for oil-based – glass jar so you can see and test on scrap wood. Use a ratio of about 2% – 3%, and test; you may need to go up to 10% to find your own desired consistency.
With the extra water, the poly will take longer to dry, but this gives the poly time to lay down before fully drying. You will find that water-based polyurethane will require more coats than oil-based poly, and be sure to let each coat dry fully before applying the next. It will still dry faster than oil-based, though.
- Don’t over-thin either type. If you do, it will not adhere as well to the wood’s surface; it will be too light in color; blotchiness could occur, and durability will suffer.
This is why you should test first before filling up the spray canister. The polyurethane container will include the manufacturer’s recommendations on thinning, and you should pay attention to them.
Polyurethane Application: Number of Coats To Use
Now that we know what polyurethane is and how it can be applied let’s look at how many coats we really need to apply for a really good finish and a professional appearance for our projects. It’s the most frequent question we are asked, and we have some practical answers for you.
We’ll distinguish between the two types of polys, as well as specific projects we might use polyurethane for as our chosen finish.
First, it’s important to understand that each type is used for a particular reason and purpose, and it depends on the project.
Oil-based polyurethane is going to leave a thicker coat and, as such, might require fewer coats. Because of its thickness, in addition to its nature, it will take longer to dry than water-based polyurethane. It will also take on an amber hue as it dries and ages, and although this can add a degree of warmth to the wood, it may not be what you are hoping for in your finish.
As we noted earlier, too, there are the VOCs, gasses emitted as it dries and cures, the smell, and the toxicity that requires a respirator on your face when applying.
For an oil-based polyurethane application, we recommend what manufacturers recommend. Three to four coats will give you a high-gloss finish, and a fifth provides just that little extra.
They know their product and have tested it extensively before bringing it to market.
While two coats might be enough, go a bit more, and when you reach the degree of gloss and color depth you want, that’s how many you will need.
Remember, though, that each coat will take a long time to dry before the next coat can or should be applied. If you are in a hurry to finish your project (no pun intended), you may not want to choose an oil-based poly.
For a water-based polyurethane application, we recommend more coats, upwards of 5 or maybe 6. This will allow the water-based poly to display its best features and properties.
You could drop down to 3 – 4 coats, but you sacrifice durability, as well as smoothness and a clear appearance.
Water-based poly is thinner on its own than oil-based, and although it dries much faster, it requires more coats to enhance its hardness and durability. It can also cause the grain to rise, and between this and the possibility of bubbles and unevenness in the early coats, will require sanding between each coat.
This will take more time, but you do want a smooth finish on your wood, and patience with the process will achieve that for you.
Once is Never Enough
You will have measured, cut carefully, assembled well, and taken your time with your project. Don’t rush the finish portion of your work. One coat of poly, whether oil-based or water-based, is never going to be enough, no matter the use of the piece you’ve made.
You want a professional appearance for your project. You want the durability of a hard coat of plastic for your project to protect it from its arch-enemy, water. While manufacturers add flattening agents to their polyurethane, they will rarely work on the first coat.
Sanding will be required for bubbles and brush marks or raised grain, blotchiness, or an uneven application. It may become less necessary with each successive coat, though, and by the time you reach a 3rd coat, you will notice a marked difference in its smoothness.
Sometimes 3 coats is enough. With oil-based polyurethane, 3 coats will often be enough no matter what it is being applied to – furniture, counters, shelves, or even cabinets. However, for flooring, you will want to plan on 4 coats, if not 5, if you are using water-based poly.
One-coat polyurethane products may not be a good choice for you, though, notwithstanding the appeal of only needing to apply just one. It’s three times as thick as regular polyurethanes and is far more difficult to apply as a result.
It’s a real chore to get a smooth first coat; no matter the polyurethane you choose, and one-coat poly products are not self-leveling, meaning you may need to be sanding those brush marks anyway.
As you’d expect, a one-coat poly is more expensive than regular polys, and if your finish will need sanding and further work anyway, it’s more prudent to save money and do the same amount of work to finish your project.
Polyurethane Applications by Project Type
We’ve mentioned some range of the number of coats and the circumstances where more or less might serve you well. Let’s look at some specific projects to see where the upper or lower range will suffice.
Cabinets. One of our favorite projects, to be honest. It’s very satisfying to build and install new cabinets in your home kitchen, and because it’s where you likely entertain from time to time, you want them to look well done and professional.
You should plan on 3 – 4 coats of poly for your cabinets. Take your time to sand between coats, and be sure to allow each one to dry before applying the next. You’ll find less sanding is required for the 3rd and 4th coats. Resist the temptation to stop after the second.
Countertops. A minimum of 2 coats, but go for 3. Again, it’s where your guests might congregate while entertaining, and you want them to look their best. Water-based poly is food friendly, and we even wrote about that in an earlier article. So, since food will likely come in contact with your kitchen countertops, choose it for your finish.
Tables. Whether kitchen or dining room, tables where food will be served and eaten should have 3 coats. Water spills, hot plates, pans, and casserole dishes will be deterred from causing harm by the hard sheet of plastic the polyurethane will provide on the tabletop.
By the third coat, too, the finish will be much smoother and give a more professional appearance.
Throughout The House
Floors. Plan on 3 – 4 coats for the best results, and don’t rule out a 5th.
Floors will get a lot of use and some abuse, whether from furniture or from kids, and the hardwood will need a bit of protection. In fact, keep applying coats until you get the finish you want, so don’t be shy about that 5th coat.
Be sure, too, that you remember all of the safety precautions we mentioned when using an oil-based polyurethane – ventilation, respirator, and longer drying times.
Stairways. The same rule applies to stairs and for the same reason. They are high-traffic by all in the household and will need extra durability. Again, go with 3 coats, and don’t rule out the fourth.
Doors, door frames, window frames, shelves, and trim. These are not likely to experience any use or abuse like floors, stairs, kitchen countertops, and kitchen/dining room tables. Plan on 2 coats for these, with sanding in between for a smooth finish. If you aren’t happy with the results of 2, you can always add a 3rd.
General Rules To Follow in Applying Polyurethane
We’ve woven general thoughts on the methods to employ when applying polyurethane as the finish on your project. Just to put a finer point on the process, follow these simple rules for the best results.
- Prep the wood. Put a smooth finish on it with adequate sanding, and be sure to clean it off – vacuum or clean, tack cloth and a damp cloth will do nicely for you.
- Not a James Bond Martini. He liked his shaken, not stirred, and that is bad for polyurethane. It will form bubbles, and that will definitely require sanding, no matter which coat you’re applying. Stir, don’t shake.
- Apply coat #1. Use any of the methods listed earlier; give it time to dry fully; sand as needed; and clean well.
- Apply coat #2, or #3, or #4. Follow the same procedure for each coat – allow to dry, sand if needed, clean, and get ready for each successive coat.
This isn’t rocket science, and if you have been working with wood for a while, you have likely worked with polyurethane. This article is intended to sharpen your understanding of polys, know how to work with them, and know how many coats of polyurethane your project will need.
Now, go get busy with your next project. And remember – have fun!