Polyurethane and epoxy are two of the most common finishes used in woodworking. Their unique properties allow them to protect and enhance wooden surfaces differently. Polyurethane offers protection against scratches, stains and UV damage while providing a smooth, glass-like coating. Epoxy provides an extremely hard, durable surface that can withstand heavy impacts.
But how do these two finishes interact when used together? Can epoxy be safely applied over polyurethane to get the best of both worlds? This article will dive into all the details about using epoxy and polyurethane. We’ll compare their properties, discuss proper preparation techniques, and consider the pros and cons of putting epoxy over polyurethane for your next woodworking project.
The Basics: Epoxy and Polyurethane
To understand the relationship between epoxy and polyurethane, it’s helpful first to review the basics of each as a finish:
What is Epoxy?
Epoxy refers to a two-part finishing product consisting of an epoxy resin and a polyamine hardener. An exothermic chemical reaction permanently hardens the resin into a solid, infusible coating when mixed.
Key properties of cured epoxy include:
- Extreme hardness and durability
- Heat resistance up to 300+ degrees Fahrenheit
- Excellent adhesion to various materials
- 100% solids composition, no VOCs
- Chemical and moisture resistance
- Lack of UV stability without additives
Common epoxy uses are coating garage floors, wood tabletops, and bar counters.
What is Polyurethane?
Polyurethanes are polymers derived from petroleum-based polyols and diisocyanates. As finishes, they provide:
- Abrasion and scratch resistance
- Stain and chemical resistance
- UV light stability
- Resistance to humidity and water
- Dries quickly within hours
- Available in oil-based and water-based formulas
Polyurethane finishes wood floors, furniture, cabinets, and other wood surfaces.
Now that we understand their unique properties, let’s consider whether they can be combined successfully.
Can You Apply Epoxy Over Polyurethane?
The short answer is that epoxy can be applied over cured polyurethane. However, there are some important caveats to consider before trying this.
While epoxy will adhere to polyurethane, it does not chemically bond to the surface. The end result is a harder but not as durable coating. Here’s a more in-depth look at how the epoxy interacts with the polyurethane:
Lack of Chemical Bonding
Since polyurethane has already cured, the epoxy cannot penetrate the surface or bond chemically. It essentially sits on top of the polyurethane layer. Even with proper preparation, the adhesion is primarily mechanical rather than chemical.
Makes Surface More Prone to Chipping/Scratches
One downside of epoxy’s hardness is it becomes very brittle and inflexible once fully cured. Polyurethane retains some flexibility and elasticity even after drying. This allows the polyurethane to resist impacts and scratches better. Covering it with a rigid epoxy layer eliminates that flexibility, making the surface vulnerable to chipping, cracks, and scratches.
Can Softens Oil-Based Polyurethane Over Time
When applied over oil-based polyurethane, the amine hardeners in some epoxy formulas can slowly interact and soften the polyurethane finish. This further reduces scratch resistance and durability. Epoxy is more compatible with water-based polyurethane in this regard.
Adds Minimal Abrasion Resistance
Since epoxy does not fuse with the polyurethane, it contributes very little if any additional abrasion resistance. Most of the scratch protection still comes from the polyurethane layer underneath.
Can Affect Clarity of Finish
Epoxy is clear but has a thicker viscosity than polyurethane. The epoxy layer can detract from the depth and clarity of the underlying polyurethane finish. This is especially true for gloss polyurethane coats.
May Need Polyurethane Top Coat for UV Protection
Unmodified epoxy yellows with sun exposure. When used outdoors, a clear coat of polyurethane may still be required on top for UV resistance.
Considering these factors, applying epoxy over polyurethane is generally not the best practice for most situations. But compatible results can be achieved with proper preparation and limitations in use.
Prepping Polyurethane for Epoxy
If you do wish to apply epoxy over polyurethane, careful surface prep is crucial for success:
Ensure Polyurethane is Fully Cured
Give oil-based polyurethane at least 5-7 days and water-based 3-5 days of cure time before applying epoxy. Insufficient curing can prevent proper adhesion. Check the manufacturer’s recommendations.
Lightly Sand Surface
Use 320-400 grit sandpaper to lightly scuff the polyurethane. This removes any waxiness and creates micro-scratches for the epoxy to adhere to mechanically. Take care not to sand through the finish.
Clean and Degrease Meticulously
Following sanding, thoroughly clean the polyurethane with isopropyl alcohol to remove all dust and oily residues from sanding. Any contaminants will impede epoxy bonding. Tack cloth can help remove dust.
Apply Epoxy Within 24 Hours
Prepared polyurethane surfaces can quickly accumulate microscopic airborne contaminants that reduce adhesion. Apply the epoxy within 24 hours of sanding for optimal bonding.
Proper prep gives the epoxy the best chance of adhering firmly. But limitations in chemical bonding and potential for diminished protective abilities remain.
Water-Based vs. Oil-Based Polyurethane
When applying epoxy over polyurethane, water-based formulas are preferable to traditional oil-based options:
- More compatible – Epoxy resins do not interact with and soften water-based polyurethanes. This maintains the protective integrity of the polyurethane layer.
- Less preparation – Water-based polyurethanes do not require extensive sanding and cleaning before epoxy application since they lack the waxy oil residues.
- Faster recoating – Water-based polyurethanes can be recoated in 2-3 hours versus 24 hours for oil-based. This allows epoxy application much sooner.
- Lower odor – Water-based polyurethanes emit far less fumes and odor during application.
For best results applying epoxy over polyurethane, a water-based poly is the top choice. Oil-based polys are more problematic but can be used with proper precautions.
Additional Considerations
Below are answers to some common questions around using epoxy with various polyurethane finishes:
Can you put epoxy over spar urethane?
Spar urethane, a variant of oil-based polyurethane, is often used for outdoor projects. But the same concerns around epoxy application remain. Spar urethane can be softened, will require extensive prep, and diminish scratch protection. As an outdoor finish, it already contains UV inhibitors, negating a main advantage of epoxy. Using epoxy over spar urethane is not recommended.
Does polyurethane bond with epoxy?
While epoxy will adhere to polyurethane with proper prep, there is no chemical bonding between the cured plastic polymer of polyurethane and epoxy resin. The adhesion is primarily mechanical, relying on roughness and cleaning rather than fusion between the products. So polyurethane does not truly bond with epoxy.
Can you put epoxy resin over polycrylic?
Polycrylic is a water-based, acrylic-modified polyurethane. It can be used under epoxy with better results than oil-based options. The water-based acrylic formula does not soften or react with epoxy resins. Properly prepared, polycrylic can provide sufficient adhesion for epoxy application, though with limitations on durability.
Conclusion
While epoxy can be applied over polyurethane finishes, doing so comes with some tradeoffs in performance, especially regarding scratch resistance and durability. The epoxy does not bond chemically with the polyurethane layer beneath it. The result is a harder but not as flexible or damage-resistant coating.
For professional quality results, applying epoxy directly to bare wood then finishing with polyurethane for UV protection will provide better adhesion and durability. Using polyurethane alone is also fine for many projects.
