If you have any questions about polyurethane, we’re pretty sure you’ll find something helpful on the subject simply by searching this site (that little magnifying glass in the upper right will help you).
Key Points:
- Yes, you can overcoat a polycrylic with polyurethane.
- Each has its strengths and weaknesses that determine where and how each should be used.
- While using just one will protect the wood surface, together, they can offer some advantages over using just one.
We’ve worked quite a bit with polyurethane, and we’ve written about it many times, answering such questions as how many coats of polyurethane you should apply, how to get rid of polyurethane smell, how to clean polyurethane from your brush, and how to speed up drying time for polyurethane, among many others.
Usually, the use of polyurethane is as a topcoat over stains or paint. Today, though, we want to discuss using poly as a second topcoat over another wood finish product – polyacrylic.
To know the answer, though, you need to understand what each is, their respective strengths and weaknesses, and any advantage to using both on the same piece(s) of wood.
What is Polyurethane?
The invention of polyurethane dates back nearly 100 years, when it was used in a number of ways, including making shoe soles, mattresses, and foam insulation. It was and continues to be used as a wood finish, as well.
We know it is non-toxic, food safe, and with the right number of coats, will create a hard and durable protective surface on the wood. It is a common choice today for a wood finish and has replaced varnish as the most common. It provides a smooth finish when applied correctly and a protective finish with the right number of coats.
It is composed of two main substances:
Polymers
Polymers can be a natural or synthetic substance that is composed of large molecules, and although that may not make much sense to you, examples will: in the synthetic category, we find nylon, Teflon, polyethylene, and epoxy; and in the natural category, we find silk, wool, protein, and DNA.
Now, we’re not giving our woodworking project a coat of Teflon, silk, or wool, but we are giving it a coating of something that will protect the wood, just as Teflon protects pots and pans. Viewing polymers as a form of plastic, it means we are coating our project with a tough, hard, and durable coat of plastic to protect it from moisture and heat.
Urethanes
Let’s keep this one a little simpler and straightforward. Urethanes are near to the family of plastics, but more so in the family of rubber. Polymerization occurs when the two substances are combined, and that process is what gives polyurethane protective properties when applied on the wood surface.
The polymers (poly) and urethane together make polyurethane.
Polyurethane History
Otto Bayer is credited with the development of polyurethane in 1937 while working at IG Farben. Among its original uses was as a coating on airplanes during WWII.
As more uses for poly were developed, its composition was adapted for them, and it became a popular and ubiquitous commercial product in the industry. Poly foams were developed, and harder forms of poly were made available for the making of furniture and clothing.
Eventually, its use as a protective coating for airplanes led to its use as a protective coating for wood in general, and today we know it in the woodworking shop as a popular and effective topcoat for our woodworking projects. Its coating on the wood surface hardens and offers effective protection from moisture and water spills and, to a degree (pun intended), from heat.
Types of Polyurethane
The mixture of polymers and urethanes needs a medium to hold it, and when it comes to polyurethanes, there are two types that we all know and use in our work. Each has its own advantages and disadvantages that depend on the type of wood being used and the use to which the project will be put.
Water-based Polyurethane
The first type is water-based polyurethane. It uses water as the medium to hold it together and carry the poly solids and is probably the one most commonly chosen by the woodworking hobbyist.
It dries quickly to a hard and durable finish but requires multiple coats to offer the type of protection we all want for our projects. While more coats can equal greater cost, there have been advances in the composition formulae for water-based poly that can reduce the number of coats for the best protection.
However, we will say that more coats of water-based poly do make it more effective in protecting against water, mold, and fungus.
Oil-Based Polyurethane
The second type of polyurethane is oil-based polyurethane which uses, as you may expect, oil as the medium to carry the poly solids. In the past, oil-based poly was considered more durable than water-based poly, but advancements in formulas have put them mostly on equal footing.
The use of oil-based polyurethane requires more safety measures be followed than does the use of water-based poly. The VOCs (volatile organic compounds) found in oil-based poly is toxic until it has dried fully (24 – 48 hours needed) and cured (30 days needed). Overexposure to them is dangerous and can lead to respiratory problems and lung damage.
A respirator should always be worn when using them, and rooms should be very well-ventilated. Even then, eye, nose, and throat irritation may result.
While many woodworkers swear by oil-based poly, most find them too inconvenient to work with, not only because of the necessary safety precautions but also because of the extended drying and curing time. They also tend to darken over time and are more at risk of denting than a water-based poly.
So, water-based or oil-based polyurethane? What’s the project, and where is the topcoat going?
These will answer the question for you.
Application of Polyurethane
Polyurethane can be applied in one of several ways, depending on your preference:
- Brush. While originally it could be applied only by brush, advancements in formulas do make other means possible. But a brush still works, although it may present problems. It is possible that bubbles will form during application that will require sanding between coats. While sanding is certainly advised, it is absolutely necessary when bubbles do form. A foam brush will mitigate against that and also will not leave brush marks.
- Rag. Wipe-on is another application method for poly. Less chance of bubbles and certainly no brush marks, and the application will be much smoother. There are even some poly products that are made specifically for a rub-on application.
- Spray. This is our preferred method of applying poly. We get greater control over the evenness of application, no bubbles or brush marks, and a smoother coating on the wood surface.
While there are some products that claim to be one-coat polys, they are 3-times as thick and more difficult to apply than either water-based poly or oil-based poly. As a consequence, we’ve never used them.
Maybe out of curiosity, someday we will see if they live up to the billing, but that won’t happen any time soon. For now, we are satisfied with 3-5 coats of polyurethane for our projects when we use a poly, and for us, we prefer a water-based polyurethane.
What is Polycrylic?
Polycrylic is a water-based interior finish for wood. It is a clear finish topcoat that offers protection from temperatures, water, and light. It needs to dry and cure first before it can withstand the heat of a coffee mug. However, it is easy to use, dries within hours, can be recoated after drying, dries completely clear, and does not show a yellowish tint over time.
Polycrylic is not waterproof, but it is water-resistant. It is not as durable as a polyurethane finish, but it is durable enough to resist scratches, dents, blemishes, and scuffs.
It is strictly a water-based product and does not have an oil-based counterpart. As such, it is environmentally friendly and will clean up easily with warm water and soap. Its VOC levels are low, as are noxious fumes emanating from its use. Still, you will want to make sure you are working with it in a well-ventilated room.
Polycrylic comes in a variety of finishes, including ultra-flat, matte finish, satin, semi-gloss, and gloss. It’s a good choice for small interior use on furniture and trim in your house and even cabinets.
Among the best-known polycrylic products is Minwax Water-based Polycrylic. Minwax Water-based Polycrylic offers all of the advantages of polycrylic, including its ease of use, quick drying time, and easy cleanup. A quart will run you around $25, depending on where you are shopping.
A thin first coat is a good way to start, with a light sanding using high grit sandpaper, which is sufficient before adding a second coat.
Which is Better For Your Interior Project: Polyurethane or Polycrylic?
It depends, actually. It has to do with what you are attempting to achieve with your project. For smaller interior projects, such as those we have already mentioned, a polycrylic does have some advantages over polyurethane:
- much less toxic than an oil-based poly
- much less noxious odor than poly
- easier to work with on small indoor projects
- dries much faster
- cleanup is easy
But poly has its strengths, too. It’s a better choice for tables and flooring, wood surfaces that will get a lot more use and abuse than smaller projects. It’s also more durable over time, and it protects better against heat and water, as well as against scratches.
Can Polyurethane and Polycrylic Be Used Together?
Yes, they can, with a coat or two of polyurethane as a topcoat to polycrylic. The better question is whether they should: does doing so add a measure of protection to the woodworking project that makes it a good idea?
Polycrylic, as we have mentioned, offers a beautiful finish in various degrees of shine to the wood surface and alone is a fine choice for small pieces that will not get a lot of use. Polyurethane, though, is tougher, more durable, and offers a greater protective coating of plastic that seals the wood from moisture and water spills.
Using them together, then, will offer a greater degree of protection to your wood and allow the clear beauty of the polycrylic coat to shine through. Yes, polyurethane will develop a yellow tint to the wood over time, and probably isn’t a good choice for a light color wood like maple. But darker woods can certainly handle the poly tint over time.
Using a topcoat of polyurethane over polycrylic joins the clarity of a polycrylic finish, no matter the sheen you choose, with the strength and durability of polyurethane and its ability to withstand scratches, dents, and scuffs.
Video Discussion of Polyurethane and Polycrylic
Here’s a video that discusses the differences and preferences between the two, although it discusses only oil-based polyurethane. Nonetheless, there is some useful information for you to consider when choosing your project finish. It’s short and to the point.
The short answer to the question is yes, you can overcoat polycrylic with polyurethane. You now know the differences and the strengths and weaknesses of each.
