Plywood is one of the most common building materials used in all sorts of interior and exterior applications, but it has its downsides. It’s prone to warping when it’s exposed to moisture, and once warped it becomes difficult to work with and weaker than normal plywood would be. The best thing you can do though is to prevent the plywood from warping in the first place.
Plywood – due to its layered veneer laminate design – is prone to warping when it’s exposed to moisture. The wood can warp very easily, but it can actually be saved in most cases provided the plywood is not too far gone.
I’ll break down the best methods of accomplishing this, so you can preserve your plywood for the long run.
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Why Does Plywood Warp?
So, what causes plywood to warp in the first place?
Moisture
The primary reason plywood warps are from absorbing water. The layers of wood will soak up moisture, particularly at the edges where the veneers are exposed. Over time this can even lead to the plywood rotting.
Plywood can absorb moisture from a leak in a roof or floor, from humidity, and from rain or snow.
Typically, what happens is one side of the plywood absorbs more moisture – which then leads to the opposite side becoming cupped. This happens because one part of the board dries faster than the other, causing the drier side to shrink faster.
Improper Storage
If you keep your plywood leaning up in a garage bowed against a wall, this can lead to bowing and flexing over time. Plywood that’s been flexed for too long will eventually take on the shape that it was in while flexed.
This will lead to a warp that can be difficult to get rid of.
Physical Damage
If the plywood is allowed to crack or split, it can lead to warped plywood at the point of impact. Obviously, you want to avoid using damaged plywood for most applications, but if you must, you’ll want to keep it from warping and bending beforehand.
How to Prevent Plywood from Warping?
How can you prevent the plywood from warping in the first place?
Proper Storage
Properly storing your plywood will go a long way to preserving its integrity and preventing it from warping in the first place. Make sure to keep it out of direct sunlight, prevent any moisture from getting into it, and store it in a manner where it’s properly supported.
The ideal way to store plywood is inside a garage, shed, or another protected environment. Use some spare 2 x 4s laid down on the floor in 2-foot intervals to support the plywood off the ground.
Another solid alternative is to store your plywood overhead in a hanging storage rack.
If you have limited space, then you can lean sheets up against the wall using ratchet straps to keep them secured together across the top, middle, and bottom.
Sealant
If we’re talking about plywood that’s already installed, then applying some type of polyurethane sealant to it will protect it over the long run. Polyurethane sealant is easy to apply and will result in a long-lasting coating that won’t wear out. It will also improve the appearance of your plywood.
To apply a sealant to plywood, you’ll want to start by sanding down the surface with 220-grit sandpaper. This will smooth out the surface and prepare it to receive the protective coating. If you’re working with grade A plywood, then the surface comes smooth and pre-sanded and you can skip the sanding process.
Apply several coats of sealant to the surface, with a quick sanding with 320-grit sandpaper in between each coat. The in-between sanding is useful for giving the next coating something to ‘grip’ onto.
After you apply several coats of polyurethane, consider applying a layer of wax over the surface. This will provide you with a high gloss shine, as well as protect the surface from dust setting on the surface.
Sealing the End Grain
As mentioned previously, plywood, like other wood products, absorbs water through its end grain. The edges will actually absorb water 12 times faster than through the rest of the plywood, so sealing it off will greatly reduce the odds of the wood warping.
You can seal off the edges in a number of different ways, including painting, shellacking, and varnishing. When applying the sealant, keep in mind the ply will absorb more sealant than you probably think. Apply continuous layers until the end grain won’t absorb any more sealant, and it’s completely saturated.
Can you Save Warped Plywood?
Even if your plywood is warped, there is a good chance you can still salvage it and bring it back to working condition.
The best method to accomplish this:
- Spray the concave (inwardly bowed) portion of the warped plywood with water. As the other side is curved, we need to reverse this curve by changing the process.
- Set down the plywood sheet on a flat surface, allowing it to receive full sunlight.
- Place a heavy object in the middle of the plywood – where the hump is, so the weight will flatten it out.
- Keep an eye on the plywood and check on it every so often for signs of flattening.
- Once it flattens out, place the plywood indoors in a cool, dry, protected spot.
