What’s the Best Way to Waterproof Plywood?


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When you’re building a shed or any other type of construction that involves plywood that will be subject to weather, you want plywood that will stand up to what nature dishes out. You want that plywood to last a long time, especially if it’s under some other materials such as when you use it for a roofing base.

While the plywood will be covered with shingles or siding, you also will be unable to check it to see how it’s holding up. Ideally, you’d like it if the plywood was waterproof. How do you accomplish this?

The top 5 ways to waterproof plywood is to use a polyurethane varnish, penetrating epoxy sealant, spray liquid latex, drying oil and waterproof paint. In this article I go over how to apply these waterproofing agents to protect your plywood from water and moisture.

What’s the Best Way to Waterproof Plywood for Outdoor Use?

First of all, you need to know how to use the product you choose. You’ll need to pick an area with plenty of ventilation. You’ll need gloves and a safety mask to keep from inhaling fumes. Lay down a tarp to protect your floor or anything else from the waterproofing or sawdust.

You’ll need a spray gun, paint rollers, foam brushes or other tools according to the manufacturer’s recommendations for the application of the product you have. You’ll also need two grades of sandpaper, medium and fine.

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You’ll need to plan for a place to stand up your panels as they dry while touching as few places as possible. You may need to do one side at a time and let it dry before turning it over to do the other side. You may want to waterproof the panel all over, then stand it up on one edge to dry, then do the remaining edge.

Try to do this job on a cooler day, if possible, as your waterproofing substance may dry too quickly for you to use it properly, or the plywood itself may heat up, causing other problems. Remember that this is a job that needs to be done thoroughly and carefully. You may not get the results you want if you try to hurry through it. Allow plenty of time, especially if you have a lot of plywood to treat.

Check over your panels, looking for grooves or rough spots. This is where getting a better grade of plywood saves work and time. Fill any defects with wood filler and let it dry, allowing the filler to hump up above the surface of the plywood so it can be sanded down later. You don’t want any gaps that may lessen the waterproofing effect.

Wipe the board around the filler with a damp rag to better help the waterproofing bond. Sand the entire panel with medium-grit sandpaper, wipe away the dust, then finish with a finer sandpaper, usually 180 grit. Again, wipe off the dust. The purpose of the sanding is to both smooth the surface of any imperfections and to roughen it up a bit to enable your waterproofing substance to stick more firmly.

Apply your waterproofing using the recommended method printed on the label. Let each coat dry the listed time before applying another coat. Don’t forget to waterproof the edges of your panels. In fact, you may want to apply an extra coat to the edges. It’s a good idea to have any cutting of the panels done first so that all the waterproofing can be done at one time. Otherwise, you’ll have to apply waterproofing just to the cut edges after you thought the waterproofing was all finished.

If using a roller, use a foam brush or chip brush to even out the texture the roller leaves right after coating. When you’re done, wait at least a full day before installing the plywood. After you install your plywood, apply another coat of waterproofing to seal the areas around your fasteners.

Any waterproofing substance should have plenty of information on the label, including recommended application methods, drying time, number of coats needed, and methods for cleaning up your tools. If you don’t see what you need to know, your supplier should be able to answer your questions. You can also look up the product on the internet, as many websites feature this information as well. if you live in a very wet area you may want to consider a plastic shed, check out my article Are There Any Advantages to Using a Plastic Shed?

The Top 5 Ways to Waterproof Plywood

Polyurethane Varnish. This is considered to be one of the best ways to waterproof plywood. It makes a really tough coating that not only stands up to moisture and water but also to scratches. These varnishes are used to coat wooden floors and stairs, and you know how much traffic and abuse those areas get.

You can apply the varnish with a paintbrush or a roller and pad. If you want to stain your plywood, that should be done first, as you won’t be able to apply any substance over the varnish. Spraying the varnish on can even be better if you are good with a spray gun and can apply it evenly.

The goal is to apply a thin, even coat. Let each coat dry thoroughly before applying the next. Your varnish instructions should indicate a preferred drying time. Several thin coats are what you want.

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Penetrating epoxy sealant. This also dries into a tough coating, and comes at the top of some lists of ways to waterproof plywood. It can even come in tinted forms. It’s usually a bit thicker than the above-mentioned varnish, so spraying it is not an option. A roller is best, rolling both with and against the grain.

Tip the wet sealant as mentioned above with a foam brush to even out the roller texture and make it smooth. Let each coat dry for around an hour, then lightly sand it before applying the next layer.

When using an epoxy sealant, it’s best to use it on a cool, cloudy day. This is because warmer temperatures will cause the mixed epoxy to cure more quickly. While this may speed up drying time, it’s not good if you mix up a batch and find out that it’s curing before you’ve even applied it all.

Another problem comes if the plywood itself heats up in the sun or warm temperatures. The plywood actually will give off gasses as the fibers inside heat up, possibly causing a large number of tiny bubbles to form in the epoxy. When it’s cooler, the plywood is more likely to absorb the epoxy into its pores as it dries, which is what you want.

Spray liquid latex. This product comes in spray cans for smaller projects and larger containers for big jobs as well. It also comes in colors so you can stain as well as seal your plywood. This product can be used with a spray gun. You may need several coats, but this type of waterproofing protects the plywood from UV rays as well as moisture.

Drying oil. Drying oil soaks right into the wood to protect it, then hardens into a solid film as it dries. Many waterproofing products already contain some form of drying oil in their formulas. It creates a protective layer around the wood and actually strengthens it.

It’s also likely to be more economical than some of the other products. Some common ones include linseed, poppy, perilla and tung oils. The only drawback is that you will need a stronger sealant for the plywood panel edges.

Waterproof paint. If you want to paint your plywood, try waterproof paint. This paint should be oil-based to seep into the pores of the wood to block both water and UV rays from the surface. You’ll also need to apply an oil-based primer and let it dry thoroughly before applying your paint. KILZ markets and interior/exterior basement and masonry waterproofing paint that should do the trick.

There are some types of bonding primers that not only make sure that the paint sticks to the plywood, but also eliminate the need for sanding. You can also apply polyurethane over the paint to really waterproof it. Ask your supplier for recommendations on both paint and what polyurethane to use over it. You may need to prime the plywood first, check out my article Do I Need to Prime a Shed Before Painting?

Is Plywood Waterproof or Even Water Resistant?

Plywood by itself is not waterproof or water-resistant. If it gets wet, it usually dries fairly quickly, and the moisture will distribute itself evenly over a large area, if not the whole panel. This is actually a good thing, as any swelling that occurs will happen over a large area.

With the moisture spread out, the drying time is lessened, and any temporary swelling is uniform. Some other substitutes, notably, Oriented Strand Board or OSB, tend to absorb moisture on the edges, causing those edges to swell up but not the rest of the panel.

This can cause fasteners to loosen and leave you with problems. Think of flooring with raised edges you can trip over. Worse, the swelling may not go down when the OSB dries. Plywood rarely swells permanently unless it’s under water for a time.

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Pressure treatment has been the usual way to treat plywood to make it resistant to moisture. There are different grades of pressure treatment, some designed just for plywood meant for outdoor use. Plywood is made from several thin layers of wood veneer glued together.

When exterior grade plywood is manufactured, waterproof glue is used. However, this doesn’t make the plywood waterproof. You’ll usually need to do a little extra work to make your plywood waterproof.

Can You Buy Waterproof Plywood Sheets?

You can buy plywood sheets that have already been waterproofed. Of course, they will cost more than buying plain plywood. You may want to price them along with the plain ones and compare the cost of buying the sheets already waterproofed and waterproofing them yourself.

If you want top-notch waterproofed plywood, marine-grade plywood is usually considered to be the best. It’s designed to be used for waterfront construction, such as boardwalks and decks. Look for plywood that’s rated BWP, or boiling waterproof. This is the type usually used on the outside of buildings, as it’s also impervious to termites and borers.

What’s the Difference Between Waterproofing and Sealing Plywood?

There are different substances you can use to waterproof plywood. Some are not considered to be sealants. A sealant is a substance that dries into a waterproof plastic-like coating that protects the plywood. There are some waterproofing substances that soak into the plywood rather than staying on the surface.

Which Type of Plywood is Best for Waterproofing?

Try to get plywood that has a smooth surface. When looking at plywood, you’ll notice that it comes in four grades. You’ll want to get grade A plywood for best results. While it is the most expensive grade, it also has smooth surfaces on both sides without any knotholes or other defects which will need filling and sanding before you apply waterproofing substances.

Will Painting Plywood Make It Waterproof?

Painting does make plywood waterproof for a time. However, unless you are using true waterproofing paint, it will need to be repainted every few years to keep up the waterproofing effects. Water-based paints rated for outdoor use are the best, as they don’t peel or crack easily, but that doesn’t mean that they don’t ever sprout cracks. You may also want to consider staining the plywood, get more information in my article Is it Better to Paint or Stain a Shed?

Is There an Alternative to Waterproof Plywood?

There are a few building materials that are considered to be alternatives to plywood. One is reinforced polyurethane foam. It’s reinforced with fiberglass. It’s lighter weight than plywood but is waterproof and strong. It comes in varying densities and is sold in 4 x 8 sheets. Some drawbacks include the release of fine glass particles when the sheets are sawed. It also can emit toxins if it burns.

Fiberboard is sometimes suggested as an alternative. Although it isn’t exactly waterproof, it’s made by mixing wood fibers with a wax resin, making the board water-resistant. The material is cut into sheets or panels which are then treated with both high heat and high pressure, making it stronger.

Fiber cement board is made with wood fibers like fiberboard, but the bonding is done with cement rather than glues or resins. It’s used in many ways including boat building. While it may not be totally waterproof, it’s lightweight and easy to work with.

How Long Will Waterproof Plywood Last Outside?

Marine-grade plywood can last from 10 to 25 years, depending on the grade and quality, and also how it’s being used. Sealed plywood usually lasts from 20 to 40 years.

What Are the Best Products for Making Plywood Waterproof?

Polyurethane varnishes come in both water-based and oil-based products. The water-based types are considered to be more durable than the oil-based kinds and are certainly easier to work with. While the oil-based kind usually leaves a sort of amber finish on the wood, the water-based products usually dry to a clear finish. You may even have a choice as to finish, whether glossy, satin or matte.

Rust-Oleum Ultimate Oil-Based Spar Urethane is at the top of some lists to waterproof plywood. The company also makes a water-based variety.

Minwax Spar Urethane and Helmsman are good water-based formulas. Both dry to a clear finish. They also make an oil-based product plus a polyurethane gel stain if you want to stain your plywood first.

Zar Exterior Oil-Based Polyurethane is a good product for many exterior uses, so you can use any left over for other projects.

Varathane Spar Urethane is also recommended and is oil-based if you prefer that type.

Pro Marine Supplies markets a good epoxy sealant. It comes in two parts which must be mixed just before applying it. Fortunately, you can get this product in a set that comes with both parts, measuring cups, nitrile gloves, foam brushes and even mixing sticks, and of course, instructions.

This product is resistant to UV rays and won’t yellow over time as some other epoxy sealants may do. People often use this product for table and bar tops, so you know it’s going to last.

For liquid latex spray, Krylon makes an exterior product that also contains a stain to give your plywood a little character. Liquid latex is often the more economical choice in many places and comes in larger containers to use with spray guns.

DAP Alex Ultra Sealant is another good product that produces less odor and is easy to apply.

KILZ markets an interior/exterior basement and masonry waterproofing paint that should do the trick. It features excellent coverage. Like any paint, however, you do need to keep an eye on it and touch it up when necessary. KILZ Premium Primer will be good for your primer coat.

Thompson’s Water seal Waterproofing Stain is great for those who want to stain their plywood, as it has stain and waterproofing all in one product. You can also get the product in a clear formula. You do need to reapply the product every one to three years, however, so keep that in mind.

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Conclusion

Waterproofing your plywood is a good idea for those uses which will subject it to outside temperatures and weather. It’s especially important for those uses which will leave it under another material, such as siding or shingles, as you won’t be able to see whether it’s getting moisture problems.

Applying waterproofing is also a task that doesn’t require any special skills. If you can wield a paint roller and some sandpaper, you can handle this. Waterproofing does take some time, especially when you consider the drying time between coats, but it saves a lot of worry in the long run. Once you have your plywood waterproof, you won’t have to worry about it again.

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