How To Apply Polyurethane To Wood: Tips And Tricks For A Smooth Finish

Chances are you’ve used polyurethane as a topcoat for at least some of your woodworking projects.  Chances are, too, you already know what polyurethane is, or at least have a general idea.  Just to make sure, though, we’ll engage in a little science first and then get to ordinary “speak” about it. 

What is Polyurethane?

Polyurethane is a class of polymers composed of organic units joined by carbamate (urethane) links. The urethane link is a functional group that contains a carbon atom bonded to two nitrogen atoms. Polyurethanes are produced by reacting an isocyanate with a polyol.

Since polyurethane contains two types of monomers, which polymerize one after the other, they are classed as alternating copolymers. Both the isocyanates and polyols used to make a polyurethane contain two or more functional groups per molecule.

Now that we’ve made this clear, let’s move on to regular language, something easier to follow.  

Invented over 100 years ago, polyurethane is composed of polymers, which can be either natural or synthetic, and urethanes, which, when united, will polymerize to create poly.  Natural polymers include such things as silk and wool, and even protein and our DNA; synthetic polymers include such things as nylon, Teflon, and epoxy.  

Urethanes are not quite plastic and not quite rubber but are something in between.  At the molecular level, these two materials, the polymers, and the urethanes, will polymerize (react, unite, and bond) to form polyurethane, and it should be obvious where the name comes from.

Types of Polyurethanes

We know there are main types of polyurethane to choose from, and the what and the where, as well as personal preference, will determine which one you will opt for in the shop.  

Water-based polyurethane 

This type of polyurethane is water-soluble and dries clear. It is a good choice for projects where a clear finish is desired, such as wood finishing.

This makes it a safer and more environmentally friendly option than traditional oil-based polyurethanes. Water-based polyurethanes are also easier to clean up, and they dry more quickly.

Water-based polyurethanes are available in a variety of finishes, including gloss, satin, semi-gloss, and matte. They can be used on a variety of surfaces, including wood, metal, plastic, and concrete.

There are benefits to using water-based polyurethane that you will want to consider when choosing the right one for your project:

• Safer and more environmentally friendly than traditional oil-based polyurethanes
• Easier to clean up
• Dries more quickly
• Available in a variety of finishes
• Can be used on a variety of surfaces

While it is milky in appearance out of the can, it goes on and dries clear. It is thinner than the second type of poly, so more coats are required. However, as we said, it dries quickly, and subsequent coats can be added after a light sanding in short order.

Oil-based polyurethane 

This type of polyurethane is oil-based and dries to a hard finish. It is a good choice for projects where a durable finish is desired, such as on hardwood floors.

Oil-based polyurethane is a type of polyurethane finish that is made with a solvent, such as mineral spirits or turpentine. This makes it a more durable and water-resistant finish than water-based polyurethanes. Oil-based polyurethanes also have a richer, more amber color than water-based polyurethanes.

Like water-based poly, oil-based polyurethanes are available in a variety of finishes, including gloss, satin, semi-gloss, and matte. They can be used on a variety of surfaces, including wood, metal, plastic, and concrete.

Here are some of the benefits of using oil-based polyurethane:

• More durable and water-resistant than water-based polyurethanes
• Richer, more amber color than water-based polyurethanes
• Available in a variety of finishes
• Can be used on a variety of surfaces

Oil-based polyurethane dries more slowly than water-based polys, though, so your project will take longer to complete.

Also, oil-based polys are much higher in VOCs than water-based finishes and, as such, are not available in all parts of the country. While it is easy to clean up water-based polys using water, oil-based polys must be cleaned up after with mineral spirits.

We’ve mentioned VOCs (volatile organic compounds that emit gasses from the poly), and these are harmful when inhaled. A respirator should be worn, as well as gloves and goggles when using polyurethane.

The room should be very well ventilated with strong air circulation, also. The odor of oil-based poly is strong and pungent, and the respirator will also spare you from that, too, along with the harmful VOCs.

Water-Based, Oil-Modified

This third category of poly also appears milky and cloudy in the can but applies and dries to an amber-hued finish just like an oil-based poly. It dries quickly as water-based poly does and works well with all woods. It is also low in VOCs, just as water-based polys are.

In addition to these common types of polyurethane, there are also a number of specialty polyurethanes available. These include polyurethanes that are designed for specific applications, such as polyurethanes that are resistant to heat, chemicals, or UV rays.

Choosing The Right Poly For The Right Job

We mentioned that the what and the where will help you choose the right poly for the right job.  The what refers to the types of wood you are finishing, while the where distinguishes between water-based polys and oil-based polys.

• Type of wood: Some polyurethanes are better suited for certain types of wood than others. For example, oil-based polyurethanes are a good choice for dark woods, while water-based polyurethanes are a good choice for light woods.  This distinction takes into account the yellowing over time with oil-based polys.
• Finish: Polyurethanes are available in a variety of finishes, including gloss, satin, semi-gloss, and matte. The finish you choose will depend on your personal preference and the look you’re going for, as well as matching the finish in the room where the project will reside.
• Environment: Polyurethanes are also available in different formulations for different environments. For example, there are polyurethanes that are specifically designed for outdoor use, such as on fencing and furniture, as well as polyurethanes that are designed for high-traffic areas like hardwood flooring. Think a spar polyurethane for that outdoor use, and an oil-based poly for flooring, according to most flooring installers.
• Application method: How do you want to apply the poly? Wipe on – there are polys made specifically for wipe-on application with a lint-free cloth; brush on – both a bristle and a foam brush will work well, too; spray on – you’ll want to thin the poly for a spray application.

Things To Remember When Applying Polyurethane To Woods

In cooking, you will often hear the phrase “mis en place,” a French expression that literally means “everything in its place.”  A good chef prepares all of the ingredients needed for the dish(es) being prepared and lines them up in the order in which they will be used for speedy access as the dish progresses.  In a professional kitchen, that order is consistent night to night so that memory kicks in and the chef will know where to reach for what he wants or needs.

In woodworking, while that phrase is not used, the concept is the same.  We prepare the wood well at each step of the project – cut right, sanded well to a smooth surface, and assembled well.  Everything in its place.  

We have six tips to keep in mind when working with polyurethane of any type:

1. Prepare the surface. Make sure the surface is clean, dry, and free of dust and debris. If the surface is dirty or wet, the polyurethane will not adhere properly.
2. Choose the right type of polyurethane. There are many different types of polyurethane available, each with its own strengths and weaknesses. Choose the type of polyurethane that is best suited for the job. For example, if you are finishing a piece of furniture that will be used outdoors, you will need to use a polyurethane that is water-resistant, so an oil-based poly will serve you well.
3. Thin the polyurethane. Polyurethane can be thick, so it is often necessary to thin it before applying it. This will make it easier to apply and will help to prevent runs.  Water-based polys can be thinned with water, while oil-based polys will need mineral spirits to thin them out.  Thinning your poly will become especially necessary if you decide on a spray application.
4. Apply the polyurethane. You can apply polyurethane with a brush, roller, or sprayer. If you are using a brush, be sure to use a natural-bristle brush. If you are using a roller, be sure to use a foam roller. If you are using a sprayer, you will want to thin the poly.  There are also poly products that are made specifically as a wipe on poly; no brush or sprayer is needed.  For those who like the tactile pleasure that comes from finishing woods, a wipe on poly will satisfy you.  
5. Suggestions for the number of coats.  Water-based polyurethane: Two to three coats of water-based polyurethane are typically sufficient for most projects. For a more durable finish, you may want to apply up to five coats. Oil-based polyurethane: Three to four coats of oil-based polyurethane are typically sufficient for most projects. For a more durable finish, you may want to apply up to six coats.
6. Sanding in between each one. As mentioned, polyurethane is typically applied in multiple coats. Allow each coat to dry completely before applying the next coat.  You will also want to sand lightly with a high-grit sandpaper or sanding block in between each coat of polyurethane, no matter which type.  After sanding, be sure to clean the wood surface well with a clean cloth.  Expect brush marks if using a brush application, and sanding will be necessary to remove them.  They can be avoided if you use a foam brush to apply or choose to spray. The number of coats of polyurethane you should apply depends on the type of polyurethane you are using, the desired finish, and the amount of protection you need.  Stir the poly in the can well but not vigorously, and never shake the can – either one will create air bubbles that will present in the application and require even more sanding.
7. Allow the proper amount of dry time. Remember that water-based polyurethane will dry quickly, while oil-based polyurethane will take much longer to dry.  Therefore, you can add more coats of polyurethane in a day with water-based than you can with oil-based. Failure to allow adequate drying time will cause problems for your project, so give it more than enough time before sanding and moving on to the next application coat. This will help to prevent the polyurethane from becoming cloudy or uneven.
8. General Tips.
   • Apply polyurethane in a well-ventilated area.
   • Use a clean, lint-free cloth or brush to apply the polyurethane.
   • Apply the polyurethane in thin, even coats.

Tips and Tricks Video

In this video, you’ll see and hear verification of what you have just read, including the importance of prep before beginning your poly application, as well as “stirred, not shaken” in the can. 

If the wood has been well-prepared, and these tips are followed, the result will be a good finish for your project.  Polyurethane finishes are durable and hard and will protect the wood from heat and water, although an oil-based poly will do a better job in that protection than a water-based poly.  Again, the right finish for the right job.

Last update on 2024-09-17 / Affiliate links / Images from Amazon Product Advertising API