Regardless of if you are working on a small, detailed epoxy piece or are creating epoxy floors, you will want to know how to complete the project. If you are looking to add a sealant or top coat, you might be considering applying polyurethane over epoxy.
Both oil-based and water-based polyurethane can be applied over epoxy, although oil-based polyurethane may leave an amber hue (while water-based will be more clear). To ensure proper application, be sure that the epoxy has completely sealed/hardened, and sand first to allow for a mechanical bond.
Whether you are using water-based or oil-based polyurethane, you will want to ensure that you apply it correctly so that you can benefit from the additional protection that polyurethane can offer for your project. Of course, not all epoxy projects will require a sealer or top coat, but for those projects that do, polyurethane can add great protection from general wear and tear as well as UV rays (which epoxy is otherwise not protected from). Let’s take a closer look.
What is the Difference Between Epoxy and Polyurethane?
Before we begin to go over application strategies (and the reasons that you may want to apply polyurethane over epoxy), it is important to understand that these products can complete similar tasks but have many key differences that make them unique.
Specifically, epoxy resin can be used for sealing wood and is much more effective for this use. Once hardened, it becomes much tougher on the exterior. Yet, polyurethane, while a bit softer and more flexible once hardened, can add additional protection against scratches and UV rays.
Consequently, it is not uncommon to see these two products used together when artists or craftsmen want to ensure that their projects have been completely sealed (and made water-resistant) but also want to ensure increased longevity. A polyurethane coat on top of epoxy can help in terms of scratch resistance and protection against general wear and tear.
Plus since epoxy resin is not UV resistant, applying a UV resistant top coat of polyurethane can help to ensure that the intended coloration of the final project remains for years to come rather than being ruined due to frequent sun exposure. Still, it is important to note that each type of product (epoxy resin and polyurethane) carry its own unique benefits for your final project.
If you are working on sealing a boat (or another type of woodworking project), then sealing first with epoxy and then applying a top coat of two-part polyurethane can be a great choice in varnish that is sure to last.
Does Epoxy Need a Sealer?
Perhaps you are new to the industry, or maybe you have been working with epoxy resin for a long time. Regardless, as you look more into applying polyurethane over epoxy and realize how common of a practice this is in certain niches, you may begin to wonder if you have been working with this (or these) products right from the start. Fortunately, it does not have to be too complicated.
Epoxy resin acts as a sealant when applied to wood, creating a water-resistant piece that is sure to stand up to general wear and tear. However, epoxy is not UV resistant, so projects that will have frequent or consistent exposure to sunlight generally last longer with a top coat (sealer).
Think about floors, for example. If you are working with garage floors or using epoxy to seal in your hardwood floors, these types of projects are going to receive a significant amount of foot traffic as well as the possibility of consistent exposure to UV rays. Because of this, you can use epoxy resin as a solid base for sealing and protecting the base of your project. However, you might want to add a sealer or top coat (like a polyurethane varnish) for the added protection against general wear and tear as well as UV rays.
Additionally, epoxy resin can leave a project with a slick, sheen appearance. While this is highly beneficial for the gorgeous and modern aesthetic that this can help you to achieve with your project, it is not incredibly practical for something that would otherwise require slip resistance (such as flooring).
Another example of this would be if you were sealing off your boat. Yes, you can apply epoxy in multiple layers to increase the longevity of the boat. After all, epoxy significantly outperforms polyurethane in terms of acting as a water-resistant sealing agent. Still, the epoxy base alone may not last as long as you’d like if your boat is going to have frequent sun exposure. In this case, you could use epoxy resin as the top-notch sealant and then apply polyurethane over top of it to gain the prolonged protection that an additional layer (that is UV resistant) can add.
Still, if you are working on other types of woodworking projects like specialty pens, coasters, or even river tables, you might not need the additional protection that a polyurethane layer could add. In this case, you would avoid applying polyurethane so as to protect the color integrity of the wood, dyed epoxy resin, or other materials used in your project.
How to Apply Polyurethane Over Epoxy
Once you have determined that the project you are working on needs a polyurethane coating over the epoxy resin base, you can rest easy that this portion of the process is not overly cumbersome. In fact, the application will be relatively similar to applying an additional layer of epoxy resin on top. You will just need to follow a few recommended steps to ensure that the two products are cohesive on your project.
To apply polyurethane over epoxy, consider the following steps:
- Determine if a polyurethane coating is necessary.
As described in the section above, it is not always necessary to apply polyurethane (or another type of sealant/top coat) over epoxy. This is highly dependent on the project that you are working on and the type of protection that it will need to be long-lasting.
So, consider the amount and type of projection that your project will need, determine if epoxy alone will suffice, and if not, consider applying a polyurethane coating carefully once the epoxy resin base has hardened.
- Decide between oil-based and water-based polyurethane.
Now, once you have determined that your epoxy resin project needs a polyurethane sealant or top boat, you will want to determine whether to use oil-based or water-based polyurethane. Again, this will depend on the type of project that you are working on.
Oil-based polyurethane is likely to offer an increased level of protection when applied as a top coat for your epoxy resin project, but it also is known for leaving an amber or yellow coloration behind. Because of this, many people (especially epoxy resin artists) prefer to work with a water-based polyurethane that should leave a more clear coating on top.
- Sand the epoxy surface to create an opportunity for a mechanical bond.
With either an oil-based or water-based polyurethane application over epoxy resin, you will need to sand the top layer of epoxy to ensure that the polyurethane is capable of bonding with the epoxy base. Without sanding the surface, the polyurethane is unlikely to create as strong of an adhesive bond with the epoxy base.
However, once you have sanded the epoxy surface, the oil-based or water-based polyurethane can bond mechanically with the epoxy base. In other words, the polyurethane particles need something to “cling to” on the epoxy base, and the raised surface from the sanding action can help the polyurethane coating to do just that.
- Test a small portion.
Of course, you will want to be careful in how you opt to sand your epoxy resin base and how high of grit you will use. Traditionally, you can use the same grit that you would have used between multiple layers of epoxy as the process of mechanical bonding is quite similar.
Along with this, you will want to test the polyurethane coating on a small, unnoticeable section to determine if this is the right type of coating for your project. This can be an excellent and helpful step to ensure that the coloration of your project is not going to drastically change based on your choice of polyurethane.
When testing a small portion, you will wand down the epoxy surface, apply a thin layer of polyurethane and then allow for it to harden. Once you have determined that this is a good match, you can proceed to apply polyurethane to the remainder of the project.
- Apply the polyurethane in thin layers.
Similar to when pouring epoxy resin, applying polyurethane over epoxy should be done in thin layers. You want to follow the product description on the label of the polyurethane that you have chosen to use. Even further, using thin layers allows for the material to harden and effectively bond creating the protective layer that you were aiming to create.
Then, if you determine that the project needs an additional protective layer over top, you can lightly sand down the top and apply another thin layer of polyurethane on that. Hopefully, you will not need too many layers of polyurethane considering you would have achieved a strong water-resistant sealant and protection from general wear and tear with the epoxy base.
- Allow to dry/harden.
Finally, after you have applied each thin layer of polyurethane over epoxy resin, be sure to allow it to fully dry/harden. This will ensure the long-term bond between the two materials as well as the overall effectiveness of the completely hardened polyurethane on your project.