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Do you sand epoxy before polyurethane?

Before putting the poly over, ensure the epoxy resin is sanded for a glossy finish. Sanding is vital because you'll remove imperfections while creating adhesion allowing a mechanical bond to form between epoxy and poly.

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Can you put polyurethane over epoxy resin? Is it safe to put polyurethane over epoxy? Which one should come first, poly or epoxy resin?

These are some of the questions that need answers if you want to have great finishes with either poly or epoxy. And luckily, this poly over epoxy resin guide will help you understand what to do.

Let’s get started.

Can You Apply Polyurethane Over Epoxy Resin Polyurethane is a good sealant, and you can confidently put it over epoxy. You’ll need two to four coats of poly to make your project shine. However, the type of polyurethane you use (whether oil/water-based) is suited for the project for them to last and perform together.

Is It Safe To Put Polyurethane Over Epoxy Resin Coatings?

Using epoxy on your woodworks protects them from wear and tear and ensures your project is moisture-resistant. However, is it safe to put polyurethane over epoxy?

Epoxy is a robust and versatile resin used for outdoor woodworking projects -because it is moisture-resistant. The only drawback for epoxy is, it has one finished form that acts as a sealant -the hardened state. This is what makes it safe to put polyurethane over the epoxy layer.

Why?

Because poly is flexible and can exist in various forms after curing. Applying polyurethane over epoxy is a perfect idea as it will protect your project from UV rays and scratches. It is safe to put polyurethane over epoxy because the poly coat on epoxy coating prevents oxidation of the vanish if exposed to UV rays, thus extending the life of the epoxy underneath. Since epoxy applied on wood helps it retain dimensions due to loss or absorption of moisture, the poly coat increases the number of layers on your woodwork, allowing it to withstand impacts.

Can You Put Water-Based Poly Over Epoxy Resin?

Yes, you can. Water-based polyurethane won’t cause problems when applied over epoxy resin coatings. However, I recommend using oil-based poly as epoxy resin glazes work perfectly on oil. Before putting the poly over, ensure the epoxy resin is sanded for a glossy finish. Sanding is vital because you’ll remove imperfections while creating adhesion allowing a mechanical bond to form between epoxy and poly.

How To Apply Polyurethane Over Epoxy (Step-by-Step)

Since epoxy goes on the woodwork before any varnish, it will be unfair if I don’t discuss applying polyurethane over epoxy. For your safety, ensure you’re working in a well-ventilated workshop, and you have the following:

Sleeves

Gloves

Respirator

Materials and Tools Needed

Here are the materials and tools you’ll need to apply polyurethane over epoxy:

120-, 180-, 220-, and 320-grit sandpaper (for brand new projects).

80-grit sandpaper (for refinishing jobs).

Sanding block/orbital sander

Oil- or water-based polyurethane

Quality brush (a natural bristle brush for oil-based polyurethane and a nylon bristle brush for water-based polyurethane)

Polyurethane aerosol spray

Automotive rubbing compound

Automotive polishing compound

Vacuum cleaner

A tack cloth or dry lint-free rags

Mineral spirits (for thinning oil-based polyurethane)

Distilled water (for thinning water-based polyurethane)

Steps-By-Step Procedure for Applying Polyurethane Over Epoxy

Step 1: Prepare the Work Area

Clean your work area to ensure it is free from dust. You should do this a couple of hours before you start working. Use a vacuum cleaner, a clean wet cloth, or a mop to clean your working surfaces.

Step 2: Ensure You Have Sufficient Lighting (optional)

Polyurethane is a clear finish, so you’ll need to set up additional lighting to help you see imperfections on your wet finish from the reflections of light.

Step 3: Prepare the Wood Surface

Wear a respirator with an organic cartridge for protection against fumes. Afterwards, if you’re refurbishing the woodwork, remove the old coatings or varnish. Use coarser sandpaper (80- or 100-grits) and sand down the workpiece. Finish it with a 220-grit to make it smooth and even. Use a vacuum cleaner or a tack cloth to dust off the wood.

Step 4: Apply Epoxy Resin

I recommend you apply epoxy primers to allow it to chemically bond. Reapply your epoxy resin on the wood piece to offer a stable base for poly. Give it 12-24 hours to dry. Let it cure, which will be seven days to offer stable base for poly.

Step 5: Apply Polyurethane Over Epoxy – First Coat

There’re several methods of applying poly over epoxy, which I’ll be taking you through.

These methods are:

Brush-on method

Wipe-on method

Spray-on method

The method of application is crucial as it will determine the final look of your project. Flat surfaces will require the brush-on method. Contoured surfaces can be finished using the wipe-on method. Use the spray-on method to finish difficult and hard-to-reach surfaces. Wipe on poly vs. brush on poly comparison will help you decide on the application method to use.

1. The Brush-On Method

To cover large and flat surfaces with the correct coat thickness, use this method. I prefer it for such surfaces because you’ll use fewer coats. This is because the brush gives you a thicker application. Reminder: Use natural-bristle brushes for oil-based polyurethane and a nylon bristle brush for water-based polyurethane.

Step I: Immerse your brush into the polyurethane about one inch.

Step II: Brush the poly on the wood following the direction of the grain. Ensure to use even, long strokes. Step III: After completing each stroke, remove any drips by running the brush over the same area. Step IV: Ensure gaps aren’t forming by overlapping half of the beginning of each previous stroke. Step V: If there’re no drips on your woodwork, allow about 24 hours for it to dry.

2. Wipe-On Technique

If you’re working on contoured surfaces, use this technique to apply poly over epoxy resin. I recommend it because it provides thinner coats allowing you to cover contoured areas easily than brushing. However, double up on the number of coats you apply on the surfaces when using this technique for an even finish.

Tip: Avoid paint drips as they tend to be difficult to remove.

Step I: Fold a piece of clean cloth into a square – about the size of your palm.

Step II: Dip the edge of the cloth into the polyurethane.

Step III: Wipe it onto the workpiece surface. Ensure you’re using even strokes and you’re applying in the direction of the grain. Step IV: Overlap the last wipe after wiping the polyurethane to avoid gaps. Step V: If there’re no drips and gaps on your woodwork, allow about 24 hours for it to dry.

3. Spray-On Technique

You must have a polyurethane aerosol spray to coat difficult-to-reach areas. These are woodworks you can’t paint with a brush or a cloth.

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Reminder: Spray in short bursts to avoid paint drips forming as they can be challenging to repair.

Protect the surrounding area if you don’t want it to be sprayed.

Step I: Flip the furniture on its top and use the aerosol spray to spray polyurethane on the bottom side first. Step II: Press the nozzle of the aerosol spray a few inches before you are over the piece. Ensure that you’re making long, continuous passes releasing a few inches after you pass the piece. Don’t start and stop in the middle of the wood as it may cause marks and drips. Prevent it by overlapping each pass 30%-50%.

Step III: Leave the first coat to dry.

Note: You need regular practice with the aerosol spray to improve your skill. Check out this detailed article.

Step 6: Wet Sanding the First Layer

Now use fine-grit sandpaper to sand the poly after it has dried. Be sure to wet the sandpaper. This is to avoid damaging the finish of the previous polyurethane. Sand it until the epoxy surface is smooth, then wipe down the surface with a wet cloth. Dry it before adding the next coat.

Step 7: Thin Your Polyurethane

Mix two parts of an oil-based poly with one part mineral spirits. For water-based polyurethane, use water. The first coat shouldn’t be thinned as it will make it less adhesive.

Step 8: Apply the Second and Third Coat

Now use any polyurethane application technique to apply the second and third coat for oil-based poly and a fourth for water-based poly. Allow 24 hours of drying time before sanding and applying each coat.

Step 9: Polish the Surface

Finally, polish the final layer of poly for a beautiful result. However, this should be after about 48 hours. Also, use a high-quality polishing paste that has a fine abrasive quality to remove any scratches.

What’s The Difference Between Epoxy And Polyurethane?

We know epoxy is tougher and has higher durability and higher compressive strength than polyurethane but what are their main differences?

What is Epoxy

Epoxy is an organic compound containing chains of carbon covalently bonded with other elements like hydrogen, oxygen, or nitrogen. Often, these elements share a pair of electrons to keep them together for adhesive purposes. The term can be used to refer to epoxy resins that appear after curing.

Properties of Epoxy

Epoxy flooring can withstand heat, water, and harsh chemicals.

It’s flexible enough to be molded into different shapes.

It has a high level of adhesion on a variety of substrates.

It’s durable and can withstand heavy loads.

What is Polyurethane

Polyurethane is a synthetic resin in which polymer units are bonded by urethane groups. Polyurethane resins are used as a constituent of many paints, adhesives, foams, and varnishes. Unlike epoxy, a compound of carbon chains chemically mixed with oxygen, nitrogen, or hydrogen, this plastic polymer is made by combining diisocyanates ( TDI and MDI) and polyols.

Properties of Polyurethane

It has a high load-bearing capacity.

It’s flexible and can be used in high flex fatigue applications.

Polyurethane is abrasion & impact resistance. Therefore, it can be used where severe wear proves challenging.

It’s tear-resistant.

Water, oil, and grease resistance capabilities.

It exhibits good electrical insulating properties.

Has strong bonding properties to bond to a wide range of materials.

Resistant to extreme temperature. Material degradation cannot be caused by harsh environmental conditions and harsh chemicals.

Cured polyurethane is Mold, Mildew & Fungus resistance

Differences Between Epoxy and Polyurethane

Many woodworkers use epoxy and polyurethane, but what are their differences?

Cost: Polyurethane costs more compared to epoxy, but it remains cost-effective in the long term. Polyurethane costs more compared to epoxy, but it remains cost-effective in the long term. Resistance: Epoxy resins have limited resistance to organic acids, while polyurethane has unrivaled resistance to inorganic alkalis, organic alkalis, corrosion, and solvents. Epoxy resins have limited resistance to organic acids, while polyurethane has unrivaled resistance to inorganic alkalis, organic alkalis, corrosion, and solvents. Tolerance to heat: Polyurethanes have superior heat, cold, and thermal shock tolerance, whereas epoxies are more rigid when exposed to heat fluctuations. Polyurethanes have superior heat, cold, and thermal shock tolerance, whereas epoxies are more rigid when exposed to heat fluctuations. Scratch resistance: Polyurethanes are generally soft and more elastic compared to cured epoxy possessing a better resistance to scratch Polyurethanes are generally soft and more elastic compared to cured epoxy possessing a better resistance to scratch Texture: Epoxy becomes chalky and brittle when it ages, while polyurethane remains smooth and glossy. For example, high end looking kitchen laminate countertops.

Why Apply Polyurethane Over Epoxy Resin?

There’re many reasons people choose to apply Polyurethane layers over Epoxy Resin, and while some of those reasons are based on personal preference, other reasons are unclear.

Some of the reasons are:

One good reason for applying Polyurethane over Resin is, it doesn’t absorb moisture well. This can be an issue if you’re using a foam cutter to cut your foam, as the moisture absorption rate can be pretty significant.

UV resistance. Epoxy will degenerate when exposed to UV light.

Polyurethane cures faster than the former adhesive. You can use poly to seal concrete floors or basement floor or wall to be ready to use the day after. In addition, curing faster ensures the coating doesn’t crack or become brittle. Polyurethane tends to bond tightly to its surroundings. This is especially true when mixed with a very thick resin. The combined thickness of the two substances makes the material much more durable than typical rigid foam.

How Many Coats of Polyurethane Should You Apply Over Epoxy?

Polyurethane coating provides a clear and durable finish for wooden surfaces. It’s used in countertops, flooring, and tables to provide a long-lasting result. It’s resistant to water, solvents, chemicals, abrasions, and impacts. It doesn’t help to strengthen the wooden surface but makes it look great.

But the question is, how many coats of polyurethane do you need to give your project an excellent finish?

Knowing the number of coats of polyurethane to apply, whether on cured epoxy or other resins, is essential to achieve great results.

Is one coat of polyurethane enough? No! Multiple coats are required.

Usually, oil-based poly finishes require fewer layers than water-based polyurethane because water-based poly is thinner.

Your workpiece will need about four to five coats of polyurethane.

On the other hand, since oil-based polyurethane has a thicker consistency, it offers excellent results after three or four coats. Note: The number of polyurethane coats can also depend on how often you’ll use the surface and the level of protection it needs.

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Epoxy with two-part polyurethane varnish

Two-part polyurethane varnish is a unique, specialty product that provides high-end protection and shine to your surface. The advantages of using two-part polyurethane varnish include its ability to withstand abrasion resistance, which will help protect the paint from peeling. The resins used have UV inhibitors to prevent further fading and provide a clear surface with no haziness. Two to three coats of epoxy will provide a good base for your two-part polyurethane finish performing a perfect beauty together.

Does Epoxy Need a Topcoat?

If you’re considering using epoxy in your project, one of the most important questions to ask yourself is, “Does epoxy need a topcoat?” It’s good to apply a top coat of polyurethane as it will give a durable, highly resistant seal that protects the structural component from moisture and weathering. While a high degree of water-resistant surface hardness of up to 20% is usually achieved with a regular epoxy application, additional sealants such as epoxy moisture barrier may be required in areas with high seasonal water activity. In areas with heavy water usage, sealing may be necessary to prevent the buildup of organic contaminants and natural airborne debris such as pollen or sugar residue that can degrade the topcoat of polyurethane. Additionally, epoxy isn’t UV resistant, so projects with consistent or frequent sunlight exposure generally last longer with a topcoat. Here is when polyurethane comes in handy.

What Paint Will Stick To Epoxy?

Many people are under the impression, epoxy paints won’t stick to anything. They are right in a sense, but it isn’t because of their viscosity. An epoxy resin coating is designed to have a thick consistency. Because of this, it can be difficult to scrape it off when you need to clean it off, which is why many people choose to go with other types of paint over epoxy. However, depending on the surface, Linear polyurethane paints, also known as LPUs, are a safe bet of paints to use over epoxy coatings. They dry hard and glossy and don’t degrade under sunlight.

Clear Coat Alternatives for Poly Finishes that Work well with Epoxy

You might ask, are there other clear coat finishes that can be applied over epoxy apart from polyurethane?

The quick answer is yes.

Poly isn’t the only finish you use for sealing wood. There’re a total of 6 best alternatives which I’ll take you through -though briefly as I’ll handle them separately and link to those articles at a later stage.

Let’s get to it;

1. Varnish

2. Shellac

3. Lacquer

4. Tung Oil

1. Vanish

Vanish is a durable poly alternative I see come out more solidly than other options in this list. I count on it to protect my furniture and other wood projects I intend to keep outside. It has a significant UV protection property meaning it won’t fade in the sun. The only drawback is, it isn’t perfect for every project as it works well for outdoor furniture and not indoor items.

2. Shellac

Shellac is among the natural finishes I love using as it is made from natural elements – a lot safer than polyurethane. See if you can polyurethane over shellac finish in case you want to seal your project. I like its warm amber color as it makes a lovely finish on my workpieces. Unlike varnish, working with shellac is easy because you can apply it to various wood surfaces -indoor and outdoor. However, the unfortunate problem is, it shouldn’t be used on wood surfaces exposed to heat because it will form white rings.

3. Lacquer

If you want a glossy finish, use lacquer.

This finish isn’t long-lasting as varnish and polyurethane are, but it gives furniture a thin coat that shines brilliantly. I have it on this list because it’s less toxic than polyurethane.

4. Tung Oil

If you want to replicate what polyurethane is capable of, then Tung Oil can be the alternative. It is a non-toxic natural finish (made from the oil of a nut) that will turn out excellently. I like it because it provides a lot of character to any wood surface I apply it to.

1. Will Polyurethane Hurt Epoxy?

No, polyurethane won’t hurt epoxy because polyurethane and epoxy bond to produce a durable finish. Yes, water-based polyurethane will adhere well to epoxy. However, you’ll need more coats of it on your wood surfaces than when you use oil-based poly.

3. What Is Stronger Epoxy or Polyurethane?

Polyurethane coat is flexible and elastic, while epoxy is harder and more rigid. With that, polyurethane is resistant to scratches and other damage, thus being stronger than epoxy.

4. Should I Polyurethane Before Epoxy?

It is possible, but the recommended way is, epoxy comes first then polyurethane. If you must apply poly before epoxy, ensure you first roughen the poly with some 100x sandpaper to make a better bond.

5. Polyurethane vs epoxy, which is better?

When comparing polyurethane vs epoxy, Poly generally offers better durability for surfaces like wood, making them the better choice between the two resins.

Conclusion

Can You Put Epoxy over Polyurethane? Yes, but it isn’t a good idea. To decide if you can put epoxy over polyurethane, you must first determine if you want a water-based or oil-based epoxy coating. Polyurethane has been around a lot longer than water-based finishes, so it’s more likely to be used for new projects. Epoxy over polyurethane provides a similar look but doesn’t have to be sealed as water-based polyurethane does. Applying epoxy over polyurethane is like putting a coat of paint on a wood plank. To get the mechanical bond, you’ll need to sand the old coating before applying poly. Epoxy over polyurethanes can be cured at room temperature, making it perfect for basement finishing projects. To apply epoxy over polyurethane, first, mix resin and epoxy with a solution. A solution of 10 percent epoxy resins and 60 percent water is good enough to provide a smooth, shiny surface. Any more than this, the resulting coating will crack, become grooved and uneven. Resin-based epoxy paints and coatings have a smooth surface and are great for basement finishing projects.

But before applying epoxy, make sure polyurethane is cured. Next read, Can You Epoxy Over Epoxy?

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