Is Polyurethane Paint Waterproof? (Explained!)

You want to use polyurethane on a surface that will see rainfall or constant exposure to moisture. But, you aren’t sure if polyurethane is waterproof? Well is it?

Polyurethane is waterproof but the paint is not 100% waterproof. This is because polyurethane coating develops microscopic holes as the paint dries.

These holes are produced when bubbles become trapped in the paint while mixing, stirring, or shaking the paint. With time, weathering effects, the expansion and contraction of the wood, and friction on the polyurethane coating cause these holes to become larger.

These holes make the polyurethane become less waterproof, and allow moisture to gradually sift in from the sides. This is why the term “water-resistant” is used to describe the moisture-repelling features of polyurethane rather than “waterproof.”

Is Water-Based Polyurethane Waterproof?

Is Water-Based Polyurethane Waterproof?

Water-based polyurethane is not waterproof but the paint is water-resistant. For paint to be waterproof, the paint shouldn’t allow water to pass through at all. However, water-based polyurethane over time allows moisture to gradually shift through the paint coating.

But you should know that water-based Polyurethane is water-resistant. The paint when dry produces a thick film that prevents moisture from passing through.

There is a misconception that water-based polyurethane is not water-resistant because the sealant is dissolved in water. This is false. Water-based polyurethane is every bit as water-resistant as oil-based Polyurethane.

It’s not the paint’s solvent or thinner that determines if it will be water-resistant or not. It’s the additives in the paint coating that determines this. Both water-based and oil-based polyurethane is produced from a combination of polyol (resin) and isocyanates or diisocyanates (hardeners). These are the additives in the paint.

The combination of these compounds creates a tough and thick plastic film that is known as polyurethane. The solvent either oil or water-based is just the vehicle or carrier of the polyurethane.

In water-based polyurethane, what keeps the polyurethane paint particles from binding in the container is water. In oil-based poly, it’s a synthetic or natural oil.

The solvent (either oil or water) has a limited effect on how water-resistant the polyurethane will be. Both solvents in fact evaporate eventually. So water-based polyurethane is not less water-resistant than oil-based polyurethane.

Let’s dive deeper into what makes polyurethane paint water-resistant.

What Makes Polyurethane Paint Water-Resistant?

The plastic compound in polyurethane is what makes the polyurethane coating water-resistant. When polyurethane cures, the combination of the additives and paint particles creates a thick plastic film. This film when dry takes on the features of a plastic material. This means the film will not be easily dented and water will not pass through, at least for the first few years.

Polyurethane paint is water-resistant because it’s a plastic finish. Polyurethane is commonly referred to as plastic in a can. The reaction of the polyol resin and the hardener creates a thick plastic film that cures over time due to the oxidization of the paint particles. When the polyurethane has cured completely, it becomes a hard plastic and as such, water can’t pass through.

Asides from being plastic, polyurethane also has a glossy top layer. The high gloss finish also repels water. Since the top layer is glossy and slick, water will easily glide off the surface. The glossy top layer particularly makes the polyurethane coating resistant to oils and grease. These substances will stick to regular dry paint but since poly is glossy, the oils, grease, and water all glide off easily.

Polyurethane paint is also water-resistant due to the flexible nature of the paint. Polyurethane tends to contract and expand based on temperature changes. When the atmosphere gets cold or moist, the polyurethane coating shrinks to tightly pack the particles together. This makes it very hard for moisture to get through. The flexibility, plastic texture, and glossy top layer of polyurethane all give the paint its water-resistant qualities.

How Long Before Polyurethane Paint Is Water-Resistant?

How Long Before Polyurethane Paint Is Water-Resistant

The polyurethane coating has to cure for the paint to become water-resistant. This will take between 3 days and 7 days.

On average, polyurethane paint becomes water-resistant 5 days after the paint was applied. You should know that the type of polyurethane, the number of coats, environmental conditions, and the application method all determine how long polyurethane will take before being water-resistant.

Before polyurethane can become water-resistant, the polyurethane coating has to reach its full potential. For this to happen, the polyurethane coating has to cure. Only when the polyurethane paint cures can it become water-resistant. When polyurethane coating has cured, the reaction between the hardener and resin would have been completed.

Also, the paint particles would have become tightly packed, hard, and impermeable. Lastly, the paint will produce a top glossy layer. All of these features combined make the polyurethane paint almost water-proof when it has cured.

Immediately the poly is put on a material, it begins to dry. After about 2-4 hours, the polyurethane will become dry to touch without being sticky.

At this state, the polyurethane is nowhere near being water-resistant. After about 24 hours, the solvent would have been completely evaporated. This is when the curing process begins. At this state as well, the polyurethane paint isn’t water-resistant.

You should remember that if you add more than 3 coats of poly or you applied the polyurethane in moist conditions, on a filthy surface, or without allowing existing coats to dry enough, it will take longer for it to become water-resistant.

Is Exterior Polyurethane Waterproof?

Exterior polyurethane is not waterproof but it’s as close to being waterproof as you can get with paint.

This means that exterior polyurethane has the best moisture-resistant qualities in the polyurethane paint family.

The reason for this improved water resistance is that exterior polyurethane is specially formulated with resins and additives to make the paint thrive in harsh outdoor conditions including heavy rainfall.

Since exterior polyurethane is used on exterior surfaces or outdoors, the polyurethane is designed to thrive in harsh weather conditions including heavy rainfall, cold weather, and snow.

To make the paint thrive in such conditions, the paint is formulated with chemicals that almost make the paint waterproof. The additives make the paint super resistant to moisture. This is why the paint doesn’t come off or blister during heavy rainfall or the winter seasons.

Does Polyurethane Make Wood Waterproof?

Does Polyurethane Make Wood Waterproof

Polyurethane doesn’t make wood waterproof because only one side of the wood is usually covered by the polyurethane coating. This means that water can still get into the wood from the other sides.

Asides from that, polyurethane itself is not waterproof. So there is no way it makes wood waterproof. However, polyurethane makes the wood resistant to moisture. The side of the wood covered by polyurethane will be shielded from spills, drips, and moisture in the atmosphere.

The polyurethane coating on the wood prevents water from getting to the wood underneath. This reduces the absorption of moisture by the wood. You should know that you’ll need multiple coats of polyurethane on the wood to make it water-resistant.

Just one coat of polyurethane is not enough because the one coat will be prone to scratches and dents. The moment dents start to appear in your polyurethane coating, the moisture resistance is drastically reduced because water can penetrate the polyurethane through the scratches and dents. So you’ll need at least 3 coats of polyurethane to make wood water-resistant.

Though polyurethane doesn’t make wood waterproof, it’s still a great way to improve the durability and resistance of the wooden surface against water and oils.

Does Polyurethane Become Less Waterproof Over Time?

Polyurethane does become less waterproof or water-resistant over time. This is because as time goes by, the polyurethane paint will wear down gradually.

Also, the contraction and expansion of the material underneath the polyurethane will cause the polyurethane to stretch and develop hairline splits that can allow water to sift through.

Not just that, the polyurethane finish will also degrade as you walk on and or place objects on it. The gradual degradation of the polymer from usage also causes the polyurethane to become less water-resistant.

However, you should know that it will take some years before any of this begins to happen. Though polyurethane gets less water-resistant over time, it will take at least 4-5 years before you’ll need to repair or paint over the polyurethane due to leaks.

In most cases, polyurethane paint will remain water-resistant for over 5 years. This is because polyurethane paint is very durable.

How To Make Polyurethane More Water-Resistant?

Though polyurethane paint is as water-resistant as you can get with any paint, there are a few hacks you can try out while applying polyurethane paint to make the finish more resistant to water and oils. Let’s check them out.

1. Apply A Polish Over The Polyurethane

You can put a polishing compound over the polyurethane to make it more water-resistant. Linseed oil and other polishing products can be used after the polyurethane has cured.

A coat or two of paint polish over the polyurethane will prevent water from getting to the polyurethane.

During spills, the water has to go through two coats of polish before getting to the polyurethane. Then the water has to go through 3 coats of polyurethane before getting to the undercoat or the material underneath.

This is next to impossible. Adding a polishing compound over Polyurethane is a good recipe for a waterproof polyurethane finish.

2. Improve The Oxidization

Polyurethane becomes water-resistant when the paint has hardened or cured and this happens through oxidization of the paint particles.

So if you improve the rate of oxidization, you improve the hardness of the paint coating and make it more difficult for water or oil to penetrate it. Improving the oxidization is very simple.

You just have to increase the amount of oxygen in the room and you can do this by switching on a few fans and opening all vents, windows, and doors.

3. Apply a Sealer Over The Polyurethane

Though polyurethane is strong enough without a sealant, adding on on the top layer of the polyurethane will improve the water resistance of the paint. Top choices include lacquer, varnish, gel, and wax. You should know that you need to sand or scuff the polyurethane before a sealant can sit on it.

Related Read: Can You Apply Gel Stain Over Polyurethane?

Final Words

Overall, polyurethane is a top water-resistant finish. The paint prevents water or oil from penetrating due to its thick plastic texture.

But polyurethane is not waterproof. Over time, the paint will allow water to pass through but this will take a few years. You can also improve the water-resistance of polyurethane by following the tips listed above.

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