Is Polyurethane Waterproof? What Tests Show

Polyurethane can be water-resistant to some extent as long as it is in good condition. Unfortunately, it can’t make wood completely waterproof. Wood is a hygroscopic material that absorbs and releases moisture, which can eventually stress the polyurethane coating and allow water to penetrate the wood.

The answer to this question is not straightforward, and several factors must be considered.

We tested how water interacts with polyurethane-coated wood to come up with this article.

Is Polyurethane Waterproof? Things to Know

Applying a solitary coat of polyurethane to wooden surfaces will create an incomplete, thin coating, which won’t waterproof wood.

Even with multiple coats, the wood may not achieve complete waterproofing, especially in harsh conditions.

Nonetheless, a thick and cohesive layer can be formed by applying sufficient coats of polyurethane, making the wood waterproof.

Although polyurethane is generally effective in making wood water-resistant, there are still limitations.

When using polyurethane for outdoor projects, it’s essential to use an exterior grade and UV-resistant polyurethane or a marine grade finish like epoxy.

water on bare vs poly-coated wood
Water on bare (left) vs polyurethane-coated (right) wood

If you observe the above test I did in my shop, it’s clear that water sprayed on bare wood did absorb into the wood pores, changing its color.

But on the polyurethane-coated wood surface, the water stayed on top of the wood without absorbing into the surface.

This test demonstrates the water-resisting property of polyurethane.

Wood is Hydroscopic

Polyurethane’s effectiveness in waterproofing wood is limited due to the wood’s hygroscopic nature.

Wood absorbs and releases moisture due to changes in relative humidity in its surroundings, which can eventually cause stress on the polyurethane coating, resulting in fractures that allow water to seep into the wood.

UV Exposure to Polyurethane

Polyurethane coated wood on sunlight
Polyurethane-coated wood in sunlight

Polyurethane has a weakness when exposed to UV rays.

In outdoor applications, the sun can damage the finish and lead to water penetrating the wood.

To prevent this, use an exterior-grade polyurethane resistant to UV rays.

Marine-Grade Polyurethane Finish

Marine-grade finishes made from specialized materials such as epoxy are recommended for waterproofing wooden surfaces that will be submerged, such as boats or surfboards.

These finishes are designed to endure extended exposure to water, making them more long-lasting and durable than standard polyurethane finishes.

Water Resistance vs. Waterproofing

It’s essential to understand that water resistance and waterproofing are distinct properties.

Water resistance means a material can repel water to some extent, while waterproofing means a material is completely impervious to water.

Although polyurethane can make wood water-resistant and somewhat waterproof, achieving complete waterproofing, especially for outdoor applications, is improbable.

You can apply additional coats of polyurethane to make the wood surface more water-resistant, but it won’t be completely waterproof.

Polyurethane Application Matters

Applying polyurethane on a piece of pine wood
Applying polyurethane on a piece of pine wood

Proper application is crucial for achieving a durable and long-lasting finish when using polyurethane as a waterproofing material.

To ensure effectiveness, the wood surface must be clean and dry before application, multiple thin coats should be applied instead of a single thick coat, and each coat must dry completely before applying the next one.

Factors Affecting Polyurethane’s Waterproofing Ability

Environmental Conditions

Humidity, temperature, and UV exposure can all impact the polyurethane finish’s effectiveness.

High humidity can prevent polyurethane from curing correctly, leading to a tacky and damage-prone finish.

Heat can cause the finish to dry too quickly, resulting in cracking and brittleness. UV rays can discolor and cloud the finish, compromising its appearance.

To ensure optimal waterproofing, apply polyurethane under ideal environmental conditions, such as a 60-80 degrees Fahrenheit temperature range and humidity between 40-60%.

Also, apply the finish in a shaded area to avoid direct sunlight, which can affect the drying process and the finish’s ability to cure properly.

Polyurethane Coat Thickness and Application Method

The thickness of the polyurethane coating is a crucial factor in its waterproofing ability.

While a single coat can provide some water resistance, it is unlikely to be fully waterproof.

To achieve the best results, apply additional coats of polyurethane, ensuring each coat is completely dry before applying the next.

The application method is also important in determining the thickness of the finish.

Applying polyurethane with a brush can create a thicker, more uniform coat than spraying or rolling, which may result in a thinner, less-even finish.

Quality and Type of Polyurethane

Fast drying wipe-on polyurethane
Fast drying wipe-on polyurethane

Different types of polyurethane have different levels of resistance to moisture and durability.

For instance, oil-based polyurethane is ideal for outdoor surfaces or areas exposed to heavy wear and tear.

In contrast, water-based polyurethane is better suited for indoor surfaces and areas that do not experience frequent moisture exposure.

Water-based polyurethane
Water-based polyurethane

Water-based polyurethane is also less odoriferous and dries faster than oil-based polyurethane.

The quality of polyurethane can also affect its ability to prevent water damage.

Lower-quality and cheaper polyurethane can result in lower protection than higher-quality and more expensive options.

It is crucial to choose a polyurethane designed for the intended purpose and follow the manufacturer’s instructions for both application and maintenance.

Final Thoughts

Water staying on polyurethane surface
Water staying on polyurethane-coated surface

Polyurethane is a useful product for protecting wood from water damage, but it has some limitations and requirements.

To get the best results, you must choose the right kind of polyurethane for the intended use, apply it in the right thickness and method, and use a high-quality product.

The wood surface must also be prepared well before applying polyurethane.

However, polyurethane is not a magic solution for wood preservation.

It still needs regular cleaning and maintenance and should not be exposed to moisture for long periods.

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Frequently Asked Questions

What happens if polyurethane gets wet?

If polyurethane gets wet, it may become damaged or compromised. Water can penetrate the polyurethane coating, causing it to peel or flake off, and moisture can seep into the wood underneath, causing it to swell or warp. Additionally, if the polyurethane is exposed to water for an extended period, it may lose its protective properties.

Does polyurethane absorb water?

Polyurethane itself does not absorb water, but the wood underneath the polyurethane coating may absorb moisture, eventually compromising the coating. If the polyurethane is exposed to water for an extended period or is not applied correctly, it may also lose its ability to repel water.

How long does polyurethane waterproofing last?

The longevity of polyurethane waterproofing depends on several factors, such as environmental conditions, thickness and application method, and the quality and type of polyurethane used. If applied correctly and maintained properly, it can last for several years, but eventually, it may need to be reapplied.