How to Keep Plywood Doors From Warping? Learn It Here!

One of the most annoying problems to deal with when working with plywood is warping. It’s a common problem when your project has to face variable levels of moisture. This usually happens with plywood doors that are put against outdoor conditions, such as shed or barn doors. The same thing might happen when they separate two rooms with different levels of heat, like an internal garage door. If you faced in the past, you might be asking yourself if there’s a way to keep plywood doors from warping.

Luckily for you, there are a few things you can do to prevent that. Since warping is mainly caused by an MC (Moisture Content) variation in the plywood, you’ll have to avoid that by sealing the outer layers of the door. You can do that with some common products.

I’ll also tell you a few things to keep in mind when building a door to minimize the chances of warping. If the damage is already there, don’t worry. A warped door can be fixed with the right approach. It will require some patience, but it’s doable. Check the paragraph I wrote about that.

What Causes Plywood to Warp?

“Plywood and Shelving” by Reuse Warehouse is licensed under CC BY 2.0

There are two types of plywood warping: twisting and cupping. If we refer to a plywood sheet, twisting happens when 1 corner is lifted in the opposite direction of the other 3. On the other hand, when all corners are lifted in the same direction, that’s called cupping. Whatever the case is, plywood warps because there are different humidity levels in it. This can be either caused by excessive heat or excessive moisture.

It’s easy to understand if you think about plywood structure for a second. Every layer that composes acts as a singularity. If the external face of a plywood sheet is exposed to excessive heat, the moisture level in the surface layer will drop, causing it to shrink. As a result, that outer layer will drag the inner layers with it, resulting in a warp. The same thing happens with excessive moisture. The surface layers will absorb it and the underlying sheets will start to move too. Basically, it’s a chain reaction.

How to Keep Plywood Doors From Warping? Follow These Suggestions

As we mentioned in the beginning, there are a few things you can do to prevent warping. The main thing is making the plywood resistant to moisture; other than that, warp prevention starts when building the door, so I’ll give you some ideas about that too.

Waterproofing the Plywood to Avoid Warping

There are many products you can use to make plywood resistant to water. Choose depending on your experience, availability, and also price: covering a whole door will require a lot of sealant. Whatever your final choice will be, don’t forget to apply it on the edges too. It’s key since plywood absorbs moisture way more there. Make sure that the wood is well sealed there. Finally, here are some products that you can choose from.

Use Varnish for Outdoor Applications

The most obvious choice is using a polyurethane-based varnish. They can be either water or oil-based. In any case, this product soaks into the wood fibers creating a thin protective layer on top, keeping water from penetrating the wood. Different finishes are available too, most common being gloss, semi-gloss, or satin.

Depending on the product you’ll choose, you’ll either have to use a paintbrush or a cloth to apply the poly varnish to your plywood door. In my experience, it’s better to use a varnish that needs to be painted. It’s easier to work on the edges with that, plus you can control the quantity you apply more accurately.

If I had to choose a varnish for this application, I’d go with Minwax Helmsman Spar Urethane. You can choose between different finishes, which is nice. In my experience, Minwax products always deliver, that’s why I’d choose that.

Seal Your Door With Shellac

Another solid choice could be to use shellac to seal your plywood door. I like to use shellac for the following reasons:

  • It’s a natural and non-toxic finish. You won’t have to deal with toxic fumes like with many other synthetic products;
  • Shellac dries very quickly. Generally, it will take around 30 minutes to dry to touch, and about 1 hour to be completely dry. It’s very handy if you want to apply multiple layers since you don’t have to wait too much;
  • It gives a nice warm look to your project; it will work for a plywood door too!

How to apply Shellac? You can either use a clean cloth or a brush. I usually use a combination of both methods: a cloth to cover most of the surface, then a brush to reach small spots that you might have left behind. Either way will work fine. Experiment with both and choose the method you feel the most comfortable with.

Give your Plywood Door a Paint Job

Lastly, you could also consider using some paint to seal your door. You will accomplish two things here: you’ll give a fresh, new look to your door; you’ll keep it from warping which is our main goal.

Careful, though. Not all paints are suited for this type of job. Make sure to use waterproofing paint, such as this Wolman Outdoor Furniture Paint.

Many products require two coats to work properly against the weather. Make sure to check the can for detailed instructions.

Building a Plywood Door That Won’t Warp

“2021-10” by biosfear is licensed under CC BY 2.0

So we learned that making plywood waterproof is key in keeping a door from warping. That’s just the last step, though. Some prevention can be done when building the door, too. It might be too late for that; in any case, keep these considerations in mind for future projects. And some of them apply to existing doors too.

  • First things first, get yourself some high-quality plywood. We’ll discuss this further later on; in short, you want pressure-treated, void-free plywood. Grade A plywood, although expensive, is a wise choice. Even better, some Marine-Grade Plywood. It should be resistant to moisture on its own; consider sealing it for additional safety.
  • Many times, warping happen because plywood has too much freedom of movement. That’s why is key to frame your door well. Consider adding some crossed reinforcements on the back. This can also be done to an existing door after it’s been flattened, to keep it from warping again.
  • If you decide to apply a sealer to your door, do it on both door faces to avoid moisture differences. Make sure to use the same product too.
  • When building a door, don’t leave it outside or on the floor. You don’t want it to be in contact with excessive amounts of moisture.

How do You Fix a Warped Plywood Door?

If the damage is already there, don’t worry. The steps to fix a warped plywood door are rather easy:
  • Take the door down;
  • Spray it with water on the concave side. Don’t overdo it: a little bit is enough;
  • Place the door on a dry surface, such as your shop floor. Consider putting something between the door and the floor to increase air circulation, such as wood studs;
  • Apply pressure on the door to help the flattening process. Use what you have available, for example some bricks;
  • Wait until the plywood is dry and check the result. Repeat the earlier steps if needed. To speed up the drying process, leave the door under the sun or at least ventilate the room where the door is. If you can’t do that, use a fan to increase airflow.
This all process will take some time, at least a few days. It will also depend on your location: drier weather will make it faster, for example. If you can’t leave the spot without a door, for example for security reasons, make sure you have a door replacement before starting the process.
Alternatively,  you could try and fix the door without taking it down. There are many ways to do this, depending on what you have available. Basically, you want to put up something on the door that will work against the warp. It also needs to hold it in place for the time needed. This works well with shed and barn doors. The changing weather conditions help stabilize the new door conformation.
As an example, check out this guy’s approach. In short, he started by placing a wire on the concave face using some hinges. Then he used a couple of wooden blocks to apply tension on the wire so that it would pull the door in the opposite direction of the warp, making it flat once again.

Plywood That Doesn’t Warp? Does it Exist?

Let me start by saying that plywood that doesn’t warp is yet to be invented. When put in harsh conditions, even the most premium plywood will warp a bit. If we are smart with it though, we can minimize or even avoid the problem. First, we need to choose a type of plywood that can fight against warping, which means it needs to be void-free and resistant to water. Marine plywood is your go-to choice for that.
Any type of pressure-treated plywood works fine against warping too since they are more likely to absorb less moisture.
If you are working on an outdoor project and you want to maximize its resistance against warping, also take into consideration the steps I mentioned earlier: frame it well, use high-quality plywood, apply a sealer to make it resistant to moisture.
At the end of the day, it all starts with choosing the right material. Consider getting A-rated plywood. If you buy quality material, it’s less likely to warp later on.

Final Thoughts

As you can see, keeping a plywood door from warping is rather easy. It just requires the right product and some patient to let it settle.

Before you apply your favorite sealant, make sure the door is in the right conditions to absorb it. Wipe it down if it’s necessary. Consider doing a little bit of sanding to open up its pores to receive the maximum amount of sealer. Make sure to remove any sanding dust. Consider using a vacuum to remove it from inside the pores, to make sure they are opened up well to receive the finish.

When using varnishes or paints, don’t forget to wear appropriate safety gear, such as gloves, glasses, and a face mask to protect you against fumes. Make sure to work in a well-ventilated area to keep fumes from building up. It will also make the sealant dry quicker, so it’s a win-win.

If your door is already warped, fixing it is not difficult at all. After you flatten it, consider waterproofing it to prevent it from warping again.

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