We know it to be a durable finish on our woodworking projects, and we’ve all likely used it in our shop. It’s a protective finish that saves the wood from heat (to a degree, pun intended) and moisture.
It sits on the wood surface rather than penetrates into the wood like other finishes and forms a high level of protection to the wood as a plastic coating impervious to water and other liquid spills.
By sanding between coats and allowing a proper and full drying time, it forms a smooth finish on our wood furniture projects in a variety of glosses. It’s also suitable for outdoor projects, too. It offers a layer of protection against the elements and preserves the integrity of the wood.
Indoor use is not limited to furniture, either; it can serve as a very good type of finish for hardwood floors where high traffic might otherwise damage the wood. In short, it has a wide range of uses in the woodworking shop and throughout the house.
But how do you know which poly is right for the job? It helps to understand polyurethane, the different available types, and the different finish options among satin finishes, gloss finishes, and matte finishes.
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.
These are pretty fancy words, true. We thought we’d get just a little technical for a moment, but here is how we have described polyurethane in previous articles, such as how many coats of polyurethane to apply.
Its invention dates back over 100 years, when it was used in the making of shoe soles, mattresses, and foam insulation. It’s non-toxic, food safe, and with multiple coats, creates a hard and durable protective surface on wood. It is a film finish in that it does not penetrate the wood; it merely forms a plastic coat over it.
The polymers (poly), natural or synthetic materials composed of large molecules, and urethane, almost plastic but closer to a rubber material, polymerize when united and together create what we use to give our woodworking projects a coating that protects against heat and moisture.
How Is Polyurethane Used?
Polyurethanes are used in a wide variety of applications. Examples of polymers include synthetics like nylon, Teflon, polyethylene, and epoxy, and in the natural category, we find silk, wool, protein, and DNA. When polymerization occurs during the bonding of polymers and urethanes, additional use categories appear, including:
Adhesives
Polyurethane adhesives are used to bond a wide variety of materials, including wood, metal, plastic, and glass.
Coatings
Polyurethane coatings are used to protect and decorate a wide variety of surfaces, including wood, metal, plastic, and concrete.
In the past, among its original uses was as a protective coating on airplanes during WWII. Its protective coating qualities, then, are not limited to just wood, although for our purposes in the woodworking shop, we are considering the right choice for the right project.
Foams
Polyurethane foams are used for a variety of applications, including insulation, padding, and cushioning.
Elastomers
Polyurethane elastomers are used for a variety of applications, including seals, gaskets, and footwear.
Molds
Polyurethane molds are used to create a variety of products, including plastic parts, fiberglass, and concrete.
Polyurethanes are versatile and durable materials that are used in a wide variety of applications. They are a major component of the modern world and are used in everything from cars to furniture to clothing.
The Types of Polyurethane To Choose From
Oil-based finishes and water-based finishes define the categories of polys, and they are exactly what those words suggest: a poly that is based in water and one based in a solvent. The differences between the two in application might help you decide which is the better choice for your project.
Water-Based Polyurethane
Water-based polyurethane is a type of polyurethane finish that is applied with water rather than with a solvent. 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
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.
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.
How Do You Know The Right Polyurethane Choice For Your Project?
There are many factors to consider when choosing the right polyurethane for your woodworking project, including the type of wood, the finish you want, and the environment the project will be in.
- 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.
- 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.
Here are some additional tips for choosing the right polyurethane for your woodworking project:
- Read the label: The label on the polyurethane can tell you a lot about the product, including the type of wood it’s best for, the finish it provides, and the environment it’s designed for.
- Ask for help: If you’re not sure which polyurethane is right for your project, ask a salesperson at your local hardware store for help. They can help you choose the right product for your needs.
- Test it out: If you’re still not sure which polyurethane is right for you, buy a small can and test it out on a scrap piece of wood. This will give you a chance to see how the polyurethane looks and feels and to make sure it’s the right product for your project.
Care and Safety When Using Polyurethane
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.
Video Quick Tips
While just a little bit repetitious, this video offers a few tips to consider when choosing which poly to use for your project. It is helpful, direct, and short.
As we have said in so many other situations, the right tool (or finish) for the right job. Only you will know what you are hoping to create with your project, whether inside or outside, and the use to which the project will be put.
The factors suggested in this article should help you decide which will best fit your needs and wishes.