How to Waterproof Plywood
Plywood is an inexpensive wood alternative – especially if you need pieces of a larger width. And if you’re putting your plywood outdoors, it’s also really easy to waterproof!
Plywood can be waterproofed with paint, polyurethane, or epoxy finish. Lightly sand the plywood with 180 grit sandpaper, clean off any sawdust, and apply the finish with a brush or by pouring the finish on if using epoxy.
That’s a general overview, but lets look a little bit deeper into the specifics of each method.
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Method 1: Waterproofing Plywood With Paint
Painting plywood is a quick and easy way to waterproof it. If your plywood will be going outdoors, exterior latex paint is the better option, while indoor projects are well-suited to interior latex paint.
Both paints will properly waterproof the plywood, so that shouldn’t be a consideration.
I’ll fully admit to grabbing some paint and slopping it on in order to get something waterproofed quickly (my little 8 ft trailer comes to mind. It’s currently covered in snow, and good news, that paint is doing a great job.)
But if this plywood project is your pride and joy, you might want to take a bit more care, which will prolong the life of both your project and the paint job.
There are two extra steps to consider: priming the plywood, and adding a topcoat.
A strong bonding primer (Zinsser’s Oil-Based Primer is my favorite) does a great job of bonding the paint to the plywood so that it doesn’t end up peeling off later.
And (drumroll please,) if you use the bonding primer, you don’t have to sand before you paint, because bonding primer is just that awesome. The whole purpose of sanding is to scratch the plywood up so that the paint has something to adhere to. The bonding primer makes that unnecessary.
A topcoat is another possibility if you’re really concerned about your plywood. Exterior paint alone really is enough to waterproof it, but if you want some extra protection in case that paint fails, adding a topcoat is the way to go.
The best topcoat for latex paint is water-based polyurethane, because it’s clear and doesn’t yellow over time. I like Varathane’s water-based poly.
Method 2: Waterproofing Plywood With Polyurethane
If you’d prefer your plywood retain a wood look, waterproofing with polyurethane is the way to go.
Interior projects actually have a ton of choices – pretty much every modern wood finish on the market will do a decent job of waterproofing plywood for the occasional splash. Take a look at this post which is all about choosing a wood finish if that’s the type of project you’re working with.
Alternatively, if you’re not into reading a 3000+ word analysis, this free flowchart walks you through the decision making process in under a minute.
Exterior projects take a lot more abuse from weather, so if your plywood will be going outside, oil-based polyurethane is the way to go.
I’d look for polyurethane that is specifically recommended for exterior use. This Spar Urethane from Varathane is oil-based, and rated for exterior projects.
I’d stay away from water-based polyurethanes, even if they advertise that they’re for exterior use.
Water-based polyurethanes are notedly less durable and protective than oil-based polyurethanes, and while that’s fine for most indoor projects, it’s not something I’d use as the sole protection on an outdoor project.
(Note that I do recommend water-based poly on top of exterior latex paint, but in that case it’s basically a bonus protection.)
Method 3: Waterproofing Plywood With Epoxy
Epoxy resin is a really thick, ultra-waterproof finish – it’s even used on boats, so you know it’s solid.
It’s also clear, so it’s another option if you’re looking for a natural wood look.
But because epoxy is so thick, it needs to be poured on, making it a difficult finish to apply to non-flat surfaces. And it’s a bit more complicated than a normal finish; most epoxy resins come in two parts, and you have to mix together a specific ratio before applying the epoxy.
If you’re considering an epoxy finish, I recommend using TotalBoat 5:1 Epoxy. I also recommend trying out the product a few times before using it on your big project. Epoxy takes a bit of practice, and you wouldn’t want the first time you tried it to be on your final project!
Method 4: Waterproofing Plywood With Plastic Sheeting
Are you actually here to figure out how to waterproof plywood underlayment that will go down on your floor? If that’s the case, none of the painting/finishing methods are a great idea – instead, you want to lay plastic sheeting to separate the plywood from any condensation that may form.
I did this last summer when laying flooring in the basement. I was laying on concrete, so I required a 2 mil plastic barrier. I bought this stuff, and it wasn’t hard at all to lay down – I placed it on the floor, then taped the seams together with duct tape.
While I waterproof plywood for woodworking projects on a regular basis, I’ve laid plastic sheeting down for floors all of once, so you should definitely read the instructions of your flooring to figure out exactly what you need to lay down as the subfloor before you start.
Non-Recommended Waterproofing Methods
There are a couple of other plywood waterproofing methods that I wanted to chat about for a few minutes, because I really, seriously don’t recommend them.
First off: penetrating oils. Pure oils are thrown around a bunch as a “non-toxic” finishing method. Which sounds nice, but pure oils don’t add much protection in comparison to the modern day polyurethanes that are now on the market.
I recommend pure oils pretty much only if you’re making a cutting board.
You’ll also see things on the market like “Walnut Oil Finish” and “Tung Oil Finish.” These aren’t actually oils; they’re thin varnishes (polyurethanes) with a little bit of oil added.
While they can actually be great for indoor projects, I don’t recommend them for outdoor projects, mostly because they’re too thin. In order to get enough coats of protection, you’d need 5+ coats, which is just annoying when thicker polyurethane is available as well.
How to Waterproof Plywood
Step 1: Fill Any Holes
The least expensive plywoods often have knot holes that should be filled prior to waterproofing. Otherwise, the change in height is an opportunity for water to get into the plywood.
To fill the knot holes, press wood filler into the hole, slightly overfilling the hole, because wood filler tends to shrink as it dries.
Once the wood filler is dry (at least 20 minutes, but maybe an hour if it’s a large hole,) sand the wood filler down to match the surrounding plywood.
Step 2: Sand the Plywood
Sand the plywood with 180 grit sandpaper. Be extra careful around the edges; it’s really easy to sand through the top veneer layer, which doesn’t look great.
If you have a variable speed orbital sander, this is the time to use it on a lower speed to help avoid sanding through the top layer. If your orbital sander isn’t variable speed, that’s okay, just press lightly and keep moving. I don’t have a variable speed orbital sander, and I’ve never had an issue.
If you don’t have an orbital sander at all, hand sanding works too. But if your project is large, maybe consider getting a sander? My orbital sander is my shop VIP, and I don’t know what I would do without it.
Cheapo sanders start around $15 at Harbor Freight if you don’t actually think you’ll sand that often.
And if you think you’ll use a sander a lot, I recommend this Dewalt sander, which is the cordless version of the one I own (and while I adore my sander, boy, do I wish it was cordless.) You can read all about my reasoning in this post.
Once you’ve sanded the plywood, wipe it down with a damp cloth to remove any excess sawdust. You don’t want sawdust to get stuck in your finish!
Finally, remember that if you’re painting your plywood, you can skip the sanding step altogether if you’re using a bonding primer.
Step 3: Prime or Stain the Plywood
If you’re planning to stain or prime the plywood before waterproofing, now is the time to do it.
I have a whole post on staining plywood, but the general gist is dip a rag into some stain, and spread the stain onto the plywood in the direction of the grain. I like this method better than the foam brush method, since I feel I have more control over how much stain actually gets on the plywood.
Whatever method you choose, make sure to wipe off any excess stain within a few minutes of applying it to the piece. Wood stain doesn’t dry, and just becomes a sticky mess if not wiped away.
Just to be extra clear: priming is a step you can do if you’re painting the piece. It’s not something you’d do under any other finish. Staining is a step you’d do under a clear finish to change the color of the wood, but is unnecessary under painting.
Step 4: Apply the Waterproofing Finish
Apply the waterproofing finish of your choice: paint, polyurethane, or epoxy.
Paint and polyurethane can both be applied with a paintbrush or a roller. Epoxy resin is typically poured onto the plywood.
Polyurethane will have brush strokes or roller marks, simply because it is so thick. To eliminate these, I sand these away after the last coat, then thin the polyurethane with mineral spirits so that I can wipe on on final, thin coat.
This coat won’t have brushstrokes because it’s wiped on with a rag, and it’ll dry quickly so dust and dirt won’t have time to get stuck in the finish.
Step 5: Let Dry and Recoat
Dry times vary between products. Paint usually can be recoated in about two hours, while oil-based polyurethane requires a full 24 hours between coats.
You’ll need at least two coats of both paint and polyurethane.
Epoxy resin dry times vary based on the hardener used. Regardless, you’ll probably only need one coat of finish.
Waterproofing Plywood Edges
I wanted to take a moment to clear up any questions you might have about waterproofing the edges of plywood, since they’re the most vulnerable part of the plywood.
Plywood edges waterproof the exact same way as the rest of the piece. As long as you’ve applied the paint, polyurethane, or epoxy to the edges, they should be waterproofed as well.
If you don’t want to see the pattern of the edges, whether the texture through paint, or through a clear finish, consider applying edge banding to the plywood.
Edge banding is a thin veneer that’s applied to the plywood with an iron. It might not be worth the time in a quick backyard project, but it’s something you might want to do on fancier indoor projects.