You want to use polyurethane on a surface that will see rainfall or constant exposure to moisture. But, you aren’t sure if it is waterproof? So, is it?
Polyurethane isn’t waterproof, but it has impressive water-resistance qualities. However, after a few years, its coating will allow water to pass through holes.
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 coating cause these holes to become larger.
These holes make the sealant less waterproof, allowing moisture to gradually sift in from the sides. This is why the term “water-resistant” is used to describe the moisture-repelling features of this sealant.
Water-based polyurethane is not waterproof but it’s water-resistant. The sealer shouldn’t allow water to pass through to be waterproof. However, this sealant type over time allows moisture to gradually shift through the coating.
There is a misconception this type is not water-resistant because the sealant is dissolved in water. This is false. Water-based is as water-resistant as oil-based polyurethane.
The paint solvent or thinner doesn’t determine whether the finish is waterproof or not, the paint additives do. Both water-based and oil-based polyurethane have the same paint additives, a combination of polyol (resin) and isocyanates or diisocyanates (hardeners).
The combination of these compounds creates a tough and thick plastic film known as polyurethane. The solvent either oil or water-based is just the vehicle or carrier of the sealant.
For water-based polyurethane, water keeps the paint particles from binding in the container. For oil-based polyurethane, synthetic or natural oil keeps paint keeps the paint particles from binding together.
The solvent (oil or water) has a limited effect on how water-resistant the finish will be. Both solvents evaporate eventually. So, a water-based sealant is as much water-resistant as a water-based sealant.
What Makes Polyurethane Water-Resistant?
The plastic compound is what makes the polyurethane coating water-resistant. When it cures, the combination between additives and particles creates a thick plastic film. This film when dry has the same features as 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.
So, this sealant 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 sealant has cured completely, it becomes a hard plastic and as such, water can’t pass through.
Asides from being plastic, it also has a glossy top layer. The high gloss finish also repels water. Since the top layer is glossy and slick, water will easily slide off the surface. The glossy top layer particularly makes the sealant 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 slide off.
Polyurethane is also water-resistant due to its flexible nature. The sealant tends to contract and expand based on temperature changes. When the atmosphere gets cold or moist, the 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 all give the sealant its water-resistant qualities.
How To Make Polyurethane More Water-Resistant?
Though polyurethane is as water-resistant as you can get, there are a few hacks you can try out while applying it to make its finish more resistant to water and oils. Let’s check them out.
1. Apply A Polish
Applying a polish compound over polyurethane makes it more water-resistant. Linseed oil and other polishing products can be used after the sealant has cured. 1-2 coats of polish will prevent water from getting to the coating.
During spills, the water must go through two coats of polish before getting to the sealant. Then the water must go through 3 coats of polyurethane before getting to the undercoat or the material underneath.
This is next to impossible.
2. Improve The Oxidization
Polyurethane becomes water-resistant when it has hardened or cured, this happens through the oxidization of the particles.
So if you improve the rate of oxidization, you improve the hardness of the coating and make it more difficult for water or oil to penetrate it.
To improve the oxidization just increase the air circulation in the room, you can do this by switching on a few fans and opening all vents, windows, and doors.
3. Apply a Sealer
Though polyurethane is strong enough without a sealant, adding a sealant over polyurethane will improve its water-resistance features. Top choices include lacquer, varnish, gel, and wax. However, you need to sand or scuff the polyurethane before applying a sealant.
Related Read: Can You Apply Gel Stain Over Polyurethane?
The polyurethane coating has to cure to become water-resistant. This takes between 3 days and 7 days. On average, it becomes water-resistant 5 days after it was applied.
The type, the number of coats, environmental conditions, and the application method all determine how long it will take before being water-resistant.
The polyurethane starts to dry immediately after it has been applied to the surface. After about 2-4 hours, the sealant becomes dry to touch without being sticky. In this state, the coating is nowhere near water-resistant.
After about 24 hours, the solvent (oil or water) evaporates completely. This is when the curing process begins. However, at this stage, the coating still isn’t water-resistant.
When polyurethane cures, the reaction between the hardener and resin is completed. The particles have become tightly packed, hard, and impermeable. After it cures, the sealant will produce a top glossy layer. All these features combined make the polyurethane almost waterproof when cured.
If you apply more than 3 coats, the sealant takes longer to cure. This is because you have to wait for each coat to dry before applying the next one. Also, if you apply it in moist conditions or on a filthy surface, the sealant takes even longer to dry, and it may turn sticky.
Does Polyurethane Become Less Water-Resistant Over Time?
Polyurethane becomes less water-resistant over time. As time goes by, the sealant starts to wear off. Plus, the contraction and expansion of the material underneath it will cause it to stretch and develop hairline splits. Water can get inside the sealant from those scratches and hairline cracks.
Polyurethane finish will also degrade as you walk on or place heavy objects over it. The gradual degradation of the polymer from usage causes it to become less water-resistant.
However, it takes a few years, usually 4-5 years, before you need to repair or paint over it due to leaks. In most cases, the sealant will remain water-resistant for over 5 years. This is because it is very durable.
Does Polyurethane Waterproof Wood?
Polyurethane doesn’t make wood waterproof. The sealant only covers one side of the wood, but water can still get into the wood from the other sides. Plus, the sealant itself is not waterproof. So. it doesn’t make wood waterproof.
However, polyurethane makes the wood moisture-resistant. The side of the wood covered by it will be shielded from spills, drips, and moisture. In addition, it will prevent water from getting underneath the wood, which reduces the absorption of moisture by the wood.
But, you need multiple coats to make it water-resistant. One coat isn’t strong enough and can easily scratch and dent. Once the sealant starts to dent, the moisture resistance is drastically reduced. Water can penetrate the sealant through those dents and scratches. So, you need 3 coats of it to make wood water-resistant.
Exterior polyurethane is not waterproof. But, it’s as close to being waterproof as you can get. This means that it has the best moisture-resistant qualities in the polyurethane family.
It has better water-resistant features because it’s specially formulated with resins and additives to make it thrive in harsh outdoor conditions, including heavy rainfall.
Since it is specially made for outdoor projects, the sealant is formulated with chemicals that make the sealant watertight. The additives’ it has, make it super resistant to moisture. This is why the sealant doesn’t come off during heavy rainfall or the winter seasons.
In summary, polyurethane isn’t waterproof but has a high level of water resistance. It prevents water or oils from penetrating its finish due to its plastic texture.
However, over time, it will allow water to pass through, but this will take a few years.
Tony is a professional painter and an author of DIY Geeks. Tony has completed over 1,000 painting projects for his clients. It's safe to say he knows what he Is talking about,