Why does wood warp when it has been protected with varnish or paint?

Wood warping is caused by uneven modification of the fibers. It is moisture that causes this phenomenon, more precisely the change in humidity. Wood is in a permanent relationship with the moisture in its environment, and changes in the moisture level cause swelling and shrinkage of the fibers. Protecting with varnish, paint, oil, wax, reduces and evens out the interaction of wood with the environment. Warping of protected objects such as table tops, interior doors, parts of furniture, stair treads, window sills, etc. is due to insufficient or unevenly applied protection. They are accentuated by the way the wood was cut, the species and the technical solution adopted.

Water in wood and the transformations caused by its inflow or outflow

To understand the relationship between wood and moisture in the air, we must first look at the water in wood. It is of two kinds, free water and bound water. Open water is what circulates in the tree along with the food. It starts flowing out as soon as the tree is cut and continues fairly rapidly until the wood reaches a moisture content of 30%, considered equilibrium or fiber saturation moisture. From here down we are talking about bound water, i.e. trapped in the cell walls of the wood. It can be released naturally until it reaches equilibrium with the humidity in the atmosphere, but much more slowly than free water, or forced out until it reaches a prescribed humidity. In the first case, we are talking about natural drying of the wood in a stack, with leaks between the pieces of wood to ensure air circulation. In the second case, the wood is dried in kilns.

The difference between the output of the two types of water is that free water only causes weight loss, whereas bound water comes with changes in (physical) properties, mechanical, technological) and internal stresses that can lead to warping, twisting and even cracking. The latter are influenced by how the wood was cut or growth defects.

The bound water content of wood is directly related to the environment in which it is found. Below is an approximate relationship between atmospheric and wood moisture content, valid at an ambient temperature between 16 and 45°C:

• 20% humidity medium - 4% humidity wood
• 40%                                 –  6%
• 45%                                 –  8%
• 50%                                 –  10%
• 65%                                 –  12%
• 75%                                 –  14%
• 80%                                –  16%

For conditions in our country, the moisture content of wood used indoors should be in the range 8-12%, and that of wood used outdoors 12-16%.

The change in environmental humidity automatically causes the moisture content of the wood to change. It absorbs or gives up moisture and the result is swelling or shrinkage of fibres. If the moisture is not absorbed evenly, the fibers deform unevenly, causing the wood to warp. In order to limit the bond with the environment and to ensure even absorption or release of moisture, wood is protected with varnishes, paints, oils or other finishing materials.

Deformation caused by insufficient protection. Solutions to reduce changes

The better the wood is coated with varnish or paint on all sides, the less contact it has with the environment. For example, a piece of wood that has been completely epoxy resin has no contact at all with the outside and thus remains permanently stable. But nobody wants wooden furniture 'drowned' in varnish or paint. To protect it and reduce its contact with the environment, thus making moisture exchange more controlled, the furniture is covered with thin layers of finish that do not destroy the wood's charm.

Furniture is usually finished assembled. Even when items are finished separately, the emphasis is on the visible parts, with the hidden parts or ends covered very little. These are the areas where moisture penetrates and causes the wooden furniture to "pop" when, for example, we open the window in winter, thus changing the level of indoor humidity.

The addition or loss of moisture will cause the fibers to deform. The construction solution chosen should not allow stresses, defects or warping to occur. For example, door planks should not be glued to the door frame as this creates tension and may crack. They are left loose in the frame, and the inside edges are painted so that the difference is not visible in the event of a warp. Reinforcement is added to the back of long table tops to prevent warping.

It is very important that the finish covers the grain end areas (where the wood is cut perpendicular to the grain) because this is where moisture exchange is greatest. Even if this exchange is not stopped, the finish makes it more uniform and reduced, thus reducing warping. Also, wood sanding before applying the finish must be properly done, with 150, max 180 grain, to regulate the absorption in the fiber.

Uneven finish application leads to visible deformation

An important cause of warping, commonly found in table tops or steps, is uneven face finishes. Typically, 3-4 coats of finish are applied to the visible side, and the back is left with a single sat chip coat with no finish. Moisture thus has different access to the wood, the fibers on the back may absorb or yield more moisture than those on the face. As a result, the panel will warp. It often happens with long table tops not fixed to the frames, or steps glued to concrete stairs with remaining moisture.

The harder the wood, the greater the stresses created. The bending of an oak step, for example, can generate forces so great that they can cause it to peel off, even if a suitable adhesive has been used. And improperly made dining tabletops can end up looking like a cucumber.

In these cases it is recommended to apply an equal number of coats on both sides, and if it is a more complicated finish, at least 2 coats of material are applied on the back, and the technical solution should be chosen in such a way that deformation is prevented (table top on the frame).

Another, and perhaps the safest, solution is to replace warp-prone timber elements with laminated solid wood. Laminated wood is much more stable, both by laying the layers perpendicular to each other and by gluing them together. But such a countertop can be much heavier, and a simplistic technical solution can lead to warping under its own weight.

I hope you find the above information useful. As always, additions are welcome. And if you have any questions or queries, please leave them below in the space provided. I will be sure to reply.