Is Polyurethane Waterproof? — All You Need to Know

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In my years of woodworking, I’ve often turned to polyurethane for its durability and finish. But one question that frequently emerges, both from budding enthusiasts and seasoned craftsmen, is if polyurethane is truly waterproof.

Let’s dive into this pressing topic and shed light on the real capabilities of this popular finish.

Does Polyurethane Make Wood Surfaces Waterproof?

Applying polyurethane does not make wood surfaces waterproof. The coating forms a protective barrier that slows down the absorption of moisture into the wood, giving you time to wipe up spills before they seep in.

However, prolonged exposure to water can still cause the polyurethane to break down over time, leading to damage and potential water infiltration.

polyurethane products

What Does Water Absorption Mean?

Water absorption can be measured by the concentration of water molecules that can penetrate a material. Furthermore, the size and number of pores dictate the rate of water absorption of the material. 

Water Absorption Testing

Simple water absorption testing is a sure way of measuring a material’s percentage of water weight gain. The testing of water absorption compares a material’s wet and dry weight for one week.

The water absorption values will differ based on the ambient temperature, polymer, length of exposure, and additives. Consistent results can be achieved through the ASTM Standard D570 test.

Also, you can assess the weak points of a material through water absorption testing.

Can Polyurethane Protect Wood from Water?

Adding a polyurethane coating to wood can protect it from elements, but it can still allow water absorption. The resulting coat still has microscopic pores on the surface, which allows some water to penetrate eventually. 

table top applied with polyurethane

Polyurethane coats are thin and thus require multiple coats. These layers reduce its porosity which will help protect the wood making it more resistant to water. Polyurethane creates a plastic finish once dried, but a solid coat creates a semi-gloss or glossy finish. 

What Happens to Polyurethane Over Time?

If you consider using polyurethane on your furniture, you might also ask yourself if polyurethane increases water absorption over time. 

As time goes by, wood expands and contracts. This wood character leads to natural abrasions, which wear the protective layer of the polyurethane over time. Furthermore, more exposure to moisture will weaken the layer, showing more weak points.   

Waterproofing Properties Compared to Other Materials

Polyurethane’s ability to absorb water is influenced by various factors, especially when compared to other finishes or materials. Given its unique physical properties and origin, polyurethane generally has a lower water absorption capacity than many other finishes or formulas.

Making Wood Waterproof Using Polyurethane

Knowing how to turn surfaces like plywood waterproof is important. Waterproofing repels water and other elements. Since poly cannot 100% protect wood from water, you must apply other formulas and processes to make it completely protected.

walnut table

Here are the steps you need to follow to turn wood waterproof using polyurethane. 

Step #1: Sanding the Surface

It’s crucial to thoroughly sand the entire exterior of the wood before applying any sealant. This not only ensures a stronger bond between the wood and the sealant but also effectively removes scratches and blemishes. Plus, a well-sanded surface significantly reduces the chances of air bubbles forming during application.

Step #2: Removing the Dust

Sanding inevitably produces a fair amount of dust. Make sure to clean off the dust using a lint free cloth or a damp rag before proceeding with the next steps. Allow the surface to completely dry before applying a sealant to it. 

Step #3: Sealing the Wood Surface

Apply your sealant [1] using a paintbrush. Make sure to use long even strokes from end to end. Catch any drips with your brush and smoothen them unto the exterior. 

Step #4: Applying the First Polyurethane Coat

Use a high quality brush, spray, or roller to apply the first coat of the sealant. Make sure that you get an even coat. 

can of polyurethane and paint brush

Step #5: Shaving the Bumps Off

Once the surface is dried up, examine the wood and remove any observable drips. You can do this using a razor. 

Step #6: Wet Sanding Your First Poly Coating

Once you have allowed it to dry for a certain time, wet sand the first application to remove any blemishes. Use circular motions when performing this step. 

Step #7: Applying the Second Coat

Once the layer is dry, you can now apply the second coat. In this step, you need to know how long you should wait between coats of polyurethane on hardwood floors, walls, roofs, etc. 

Step #8: Applying the Final Coat

It is not enough to have just one layer. It will take additional coats or about three coats. Make sure to the first coat to dry completely before applying the next. Refer to the instruction guide to determine the recommended drying time of polyurethane

applying Varathane 200241H Water-Based Ultimate Polyurethane to table

Step #9: Polishing the Wood Surface

Polish the exterior after at least forty-eight hours. 

You might be interested in: Ways to Waterproof Wood for Bathroom Floors & Walls

How to Make Wood Waterproof: 3 Methods

If you’re aiming to prolong the beauty and lifespan of your pieces, waterproofing is the way to go. I’ll walk you through some tried-and-true methods and the best corresponding waterproofing products to ensure your furniture stands the test of time.

Method #1: Use Sealants

Different types of products are sealants for waterproofing wood. Some products, such as polyurethane, lacquer, and varnish, are proven to work. They have excellent waterproofing properties. 

Sealants are considered finishing items that have very good water-resisting qualities. Consider each type of sealant for the specific application you intend the product to be subjected to. 

Polyurethane sealants have polyurethane and acrylic resins in combination with other solvents. The current formulation will not give a yellow color on the wood. Oil-based variants provide the greatest durability; however, they require turpentine or mineral spirits during clean-up. 

sealant applied on wood

Varnish results in a hard finish that is resistant to small scratches and does not yellow over time. It is best to apply marine varnish to outdoor furniture. It is formulated to contain UV absorbers that resist sun destruction. This keeps wood protected for outdoor use. 

Lacquer is a formulation that involves natural resin and synthetic resin in alcohol. It gives yellow discoloration over time. Despite this, lacquer is the sealant of choice among many. It brings out a warm, scratch-resistant coating on the wood. 

How to Use Sealants

  1. Stir the sealant but do it gently. 
  2. Apply the sealant on the surface as even as possible.
  3. Allow the coat to dry
  4. Sand the exterior once it is dried out
  5. Clean off the dust
  6. Repeat these steps until you get three coats of the sealant

Method #2: Apply Wood Oils

Using wood oils such as linseed or tung oil is effective in making wood waterproof. Linseed oil is extracted from the flax plant, whereas the Tung oil is from the Chinese Tung tree. Applying wood oil can also lead to a beautiful coat that restores the wood’s natural beauty. 

tung oil

You can get pre-blended Tung oil and Linseed oil or make your own custom finish. Here are the steps for applying wood oils to the wood surface. 

  1. Stir the oils thoroughly.
  2. Using a natural-haired paintbrush, apply a thin layer of the oil to the wood that has been cleaned and sanded down. 
  3. Allow the oil to be soaked unto the surface of the wood. Examine the surface and reapply the oil on the exteriors that look dry.
  4. Remove any excess oil from the surface.
  5. Make sure to give ample time for the oil to dry. 
  6. Sand the exterior with a fine grit sandpaper
  7. Repeat the process to get the coat number necessary

Method #3: Use a Stain-Sealant Combination

A third method to waterproof wood is by applying a stain sealant combos. The stain sealant combo can be used especially in cases when you have limited time and you have a large area to cover. 

foam brush and Waterlox TB5284 Stain

Stain sealant is formulated to have color pigments and binders. These binders are either water, alkyd, or oil based. The resulting exterior is either opaque, transparent or anything in between. Make sure to reapply the stain sealant annually or every two years. 

  1. Stir the stain-sealant combination thoroughly.
  2. Using a natural bristle paintbrush, apply a thin layer of the mixture to the wood that has been cleaned and sanded down. 
  3. Allow the mixture to be soaked unto the exterior of the wood. Examine the exterior and reapply the oil on the exteriors that look dry.
  4. Remove any excess mixture from the exterior
  5. Make sure to give ample time for the mixture to dry. 
  6. Sand the exterior with fine-grit sandpaper. 
  7. Repeat the process to get the coat number necessary.

Making Hardwood Floors Waterproof: 7 Effective Ways

Using hardwood for your next project does not guarantee that it is waterproof. Here are seven effective ways to turn hardwood waterproof. 

milled Cherry wood

Tip #1: Choose Harder Woods

Plan your woodworking project to include hardwoods as the raw material for construction. Hardwood floors have water-resistance properties since their fibers are packed more closely together. 

Whereas softwoods such as bamboo are porous and absorb more water. Thus, they do not have water-resistance properties. 

Tip #2: Opt for White Oak Instead of Red Oak

Make sure to choose White oak over Red oak. Due to its cellular anatomy, white oak is more resistant to water than other oak types. The white oak’s cellular structure consists of pores with tyloses, making it water resistant. Furthermore, its wood fibers are compact. 

Due to White oak’s cellular structure, it is commonly used in canoes and boats. If you use white oak for your home, place it in areas prone to moisture and high humidity, such as comfort rooms and kitchens. 

Tip #3: Add Multiple Layers of Polyurethane

It is also best to add multiple layers of polyurethane on the exterior. The more layers you add, the more protection it can provide. Three coats are enough. Too many coats can make the exterior yellow. 

(But, you might wonder if you can apply polyurethane on chalk-painted surfaces. Read next!)

applying polyurethane

Tip #4: Seek Professional Help in Sanding and Refinishing Hardwood

Not all wood projects can be done as a DIY job. Seek professional help in sanding and refinishing hardwood. This route leads to additional costs, but it is worth every penny. Professionals have years of experience and skills and use top-of-the-line machinery. 

Tip #5: Screen and Recoat the Floors Periodically

I can’t stress enough the importance of preventive maintenance to enhance durability. Often referred to as screening and recoating or simply buffing, this process is crucial. It’s not as daunting as it might seem – you can typically wrap it up in just a day’s work.

Tip #6: Wipe Off Spills

Make sure to wipe off spills from the exterior immediately. Clean the surface as often as you can. And if ever the substance get on you skin, make sure to wash polyurethane off your hands, as well. 

white rag

Tip #7: Don’t Just Rely on Waxes or Products to Restore Sheen

Do not apply waxes or products to restore the sheen of your wood surface. These products may result in a glossy finish, but this product can degrade the polyurethane on the floors. If you need to restore the surface’s sheen, you can use a regular hardwood clear. 

You might be interested in: Recommended Polyurethanes for Outdoor Applications

FAQ

Can you make wood waterproof by staining it?

You cannot waterproof wood by staining it. Staining the exterior of wood can only give the surface a certain color. 

Are water-based polyurethanes waterproof?

Water based polyurethane is not waterproof. However, water-borne poly makes wood water resistant. 

Are oil-based polyurethanes waterproof?

Oil-based polyurethane is not waterproof. Oil-based poly is water resistant. 

Are polyurethane foams waterproof?

Polyurethane foams are waterproof up to a certain extent. You need to consider certain factors, such as material, to assess whether it allows water penetration or not. 

Are urethanes waterproof?

Urethanes are waterproof to a certain extent. 

Do shinier floors mean they’re more durable and waterproof?

Shinier floors do not mean that they are more durable and waterproof. This is a common misconception claimed by a lot of people. Different sheens are all equally durable.

Is varnish waterproof?

Varnish is a tried and tested sealant making a material waterproof. 

Conclusion

Now, is polyurethane waterproof? Simply, it can decrease the water absorption rate and repels standing water. Since it’s always best to give your wood the best protection from the elements, adding oil or water-based poly on furniture is a common practice. 

I hope this article clarified this topic and helped you understand the benefits of polyurethane to woodworking.

robert headshot

Robert Johnson is a passionate furniture maker & carpenter, sought after for his knowledge on the craft.
You’ve probably seen his down-to-earth wisdom in USA Today, Bobvila, Family Handyman, and The Spruce, where he has shared commentary and guidance on various woodworking topics.

Robert is the brain behind Sawinery, where he aims to share tips, tricks, and a passion for all things carpentry.

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