Plywood is a versatile invention, allowing stylish finishes at a fraction of the cost of rare wood. It is amenable to synthetic veneers. But warped plywood kills the value proposition. Knowing how to unwarp bent wood is a handy skill, as it can preserve sunk costs and avoid the expense of having to replace the Woodstock.
Warped plywood is straightened by reversing the process that bends it in the first place: the application of excessive heat and moisture. By wetting and heating the concave side of the warp, the bend can be reversed. Time and pressure are added for extra effect while avoiding the risk of damage.
We’ll look at several methods for straightening warped plywood, each of which is readily available to the homemaker.
Why Does Plywood Warp?
The methods for straightening plywood hijack the factors that cause the wood to warp. Because of this, an understanding of the causal factors is critical. Another reason is that prevention (which we address below) requires that the factors that cause warping are managed down.
The four factors that cause the plywood to warp are:
- Variable exposure to light.
- Variable exposure to heat.
- Variable exposure to moisture.
- Exposure to insect activity.
Plywood comes in different varieties, all of which have some propensity to warping. Essentially, plywood warps because one side of the wood is shrinking faster than the other. This causes a bend that is concave on the side, contracting faster.
The interaction between sunlight, heat, and moisture contributes to this shrinkage. Suppose the wood sides have different exposure to these elements. In that case, the side drying more slowly (being more exposed to moisture and less exposed to sunlight, relative to the other side) will contract. The side in the sun will form the convex bend.
Next, it is important to regard the composition of plywood. Plywood is constructed by laterally gluing together thinner sheets. Quality plywood will have the glue thinly and evenly spread across the entire surface area connecting the layers.
The composition and application of the glue will impact the tendency to warp. Weak glue that has been unevenly applied will do a poorer job of offsetting the forces of contraction and expansion outlined above. The areas where glue is concentrated will create pressure points that form warps.
Leaning the plywood upright during storage will create variable compaction in the wood. This is because gravity places more pressure on the wood sections closer to and in contact with the floor. These will also be more exposed to moisture as water trickles downward.
Storing plywood in a context where the board sections have various exposure to the elements (specifically sunlight and ambient moisture) will lead to variable exposure to contraction factors, causing warpage as described above.
Types Of Warping
Knowing what kind of warp you have will help pick a method for addressing the damage. The factors that lead to damage create these distinctive types of bend:
- Cup: This is the typical plywood warp. It involves the wood dropping in the center to form a cup-shaped depression.
- Bow: This is similar to cupping, except that the curvature is distributed over the length of the board, not concentric around a point.
- Crook: This is similar to bowing, except that the curvature runs along one edge (a contracted edge), leaving the planar face flat but the edges curved.
- Twist: This is a warp in two dimensions, combining a bow and crook.
- Kink: Kinking is a sudden, local disruption in the evenness of the board. This is a rare type of deformity in plywood.
Five Methods For Straightening Warped PlyWood
The different methods of straightening wood can be selected based on the damage done. A broader bent surface area with more extreme curvature requires more deliberate treatment, as indicated below.
Use this method with thin plywood that has a single warp or multiple bends which are convex on the same side of the board. Start by immersing sponges or tack cloth in hot water. Then sponge the concave surface of the warps(s) thoroughly, allowing that side of the board to soak up moisture.
Now place the board with the wet concave surface facing down on a firm outdoor surface exposed to the sunlight. This differential exposure to light and moisture will prompt corrective warping in the opposite direction.
Check on the progress every twenty minutes. If necessary, reapply moisture when after an hour, there still are warps.
Similar to the sunlight method, ironing involves an interplay of differential moisture and heat. In this case, the heat is brought by a steam iron. The steam is critical as it compensates for the moisture lost through drying.
Bring the iron to heat at its highest temperature. Leave it aside for five minutes that the heat may reach its peak. While this happens, soak enough rags in warm water to cover the board’s surface.
When ironing, cover the entire board with water-soaked cloths. These must be applied to the convex as well as concave sides of the board and must cover the board entirely. Again, place the board on a firm surface with the convex (humped) side facing up.
Now iron out the warp. The method is to place the iron on the part of the warp for ten seconds and then move onto an adjacent area. Allow overlap between the areas and proceed systematically to cover the entire humped area.
After the first pass, remove the rags to consider progress. The warp should be almost gone. Having a go at the process might two to three more times may be required to get the bend completely straightened.
The sunlight method can be inverted by applying pressure to the concave hump. This can be done by applying clamps after the bulge has been prepared. Be sure to steady the weights, keeping them in place.
To begin, ensure that there is light and heat in the application area. Doing this in cold, wet conditions is ineffective. Lay the plywood on a hard surface with the concave side facing upward.
Apply wet cloths, as in the sunlight method, to the humped area. It is advisable to cling-wrap the towels to the wood to prevent evaporation and ensure that the application of moisture is limited to the warp.
Now clamp the board onto a flat, hard surface. This method takes a while. Inspect the towels from time to time to ensure that the moisture control is intact.
Jointers are purpose-built machines for flattening wood that has been cupped or curved. You may hire one if you have a lot of warpage to repair. The typical use case is for a bend along the edge of the wood, but it can also be used to realign the wide face of the plywood. Proper technique should be applied.
The use of proper safety equipment and firm adherence to machine operation instructions is critical. When straightening an edge that is warped, the outfeed table should progressively guide the wood as it passes through to ensure consistency of cut.
When the warp is on the face of the plywood, pass the convex edge incrementally. This should be done only for shallow warps on thick ply. Thinner plywood with wider warps should be treated using the other methods.
Running plywood through a jointer on edge risks nicking the blade as it makes contact with the glue line. Damage of this nature can be limited by moving the jointer’s fence to the back or front of the table. Cutting through the glue line as you joint the face of the wood will dull the blades immediately.
An alternative to separately setting plywood boards is to glue them into panels. This is suitable when the warped wood is in the shape of a bow, which can be shaped into an edge and flattened for gluing. Gluing the panel will create a large stable surface more conducive to the removal of warps.
Take care not to use clamps to force bows and twists against the glue joints in a vertical position. Clamped pressure in this position is prone to create tension within the panel. While the tension will initially be invisible, it will lead to weaknesses that emerge later.
How To Prevent Plywood Warping
Since prevention always bests cure, it is worth considering the steps to avoid having to deal with warped plywood at all. These involve careful shopping for quality plywood, sealing the wood conscientiously, and storing the stock in conducive conditions.
Safe Procurement Of Plywood
The store or brand where your wood is bought does not primarily affect the risk of bending, but looking out for known quality defaulters can save subsequent regret. If you run into a negative customer review, shop elsewhere if possible.
Armed with a knowledge of the proper storage conditions, weed out suppliers who don’t comply. Avoid buying from shops that store the plywood upright in damp, leaky storerooms with single windows that distribute sunlight unevenly. Remember that the wood spends most of its life in the storeroom – not the shelf.
Carefully inspect the plywood that you intend to buy for any damage to the wood. Such damage indicates that the wood has been exposed to the risk factors for warping. Use spirit levels and a keen eye to inspect the flatness of the wood before you buy it.
Notable too is that there are different types of plywood. Bamboo plywood has a reputation as less prone to bending. Boat-building ply has been designed to be heavily moisture-tolerant. It should be fancied for situations in which exposure to liquid is inevitable.
If you do end up buying wood that turns out to be bent, rather than attempt to straighten it yourself, head back to the shop for a swap.
Safe Storage Of Plywood
Keep plywood in a cool, dry room where the plywood is lain horizontally, free of pressure, away from moisture sources, and without exposure to changes in sunlight. Overlaying a thin, breathable tarp is a good way of minimizing light exposure.
Humidity in the storage area should range between 40%-50%. Preferably store the plywood on shelves off the ground. This will reduce the chance of contact with moisture gathering on the floor. Control the room for woodboring insects.
Do not stack objects or other stock on top of the plywood, as this leads to pressure differentials. Where possible, minimize the storage time altogether. Ideally, the wood should be acclimated, prepped, and used straight after purchase.
Acclimation takes about seventy-two hours. During this time, the wood gets a chance to adapt to the environment in which it is deployed. Acclimating before build reduces the possibility of having to dismantle an artifact in order to deal with wood that bends after the fact.
Avoid placing the plywood panels in extreme environments. Proximity to air conditioners, heaters, and humidifiers bring on the warp. Also, avoid applications that have a barrier between dry and humid spaces.
Store the plywood panels with the top and bottom of the stack covered. This slows the flow of water through the exposed surface, creating an even distribution of moisture.
While it is possible to flatten plywood that has been warped, damaged timber has a higher likelihood of warping again. It is advisable to reuse straightened plywood only in areas where it can be nailed down to prevent recurrent bending.
If you are fixing a piece of wood from the user-facing surface of a finished item – where nailing is not possible – it is best to replace the bentwood with fresh ply. The straightened section can then be recycled for background use in a later project. Or you could swap an intact piece of ply from the same finished item.
When picking a method for straightening plywood, have regard to the thickness of the wood and the extent of the warper. The thinner the wood and the milder the warp, the less need there is for pressure, and the less moisture should be applied.
As ever, prevention pips cure. Take care to shop carefully to avoid buying defective wood, and then store it under cool, dry conditions to prevent warping altogether.