The normally-visible side of a veneered panel is often called the "face" side and obviously it is an important part of the project. Unfortunately, most woodworkers don't realize that both sides of a panel must be veneered in order to avoid warping. The veneer used on this unseen side is called a "backing" or "balance" veneer. This guide will explain the what and why of using backer veneer.
Backer veneers are usually lower grade sheets that are inexpensive and easy to work with. They are critical to achieving a perfect panel. A backer veneer does not have to be a lesser grade veneer if the back of the panel will show. In that case, most craftsmen would choose to use a veneer of similar color, grain, and species. However when the back side of the panel will be unseen, a backer grade veneer is the way to complete the panel without spending a fortune.
Why Backer Veneer is Needed
In most parts of the world, ambient humidity fluctuates from season to season. Changes in humidity cause wood cells to expand and contract which can cause the entire veneered panel to grow and shrink as well. The movement occurs even when the panel has a urethane, lacquer, or other finish applied.
A backer veneer is also called a balance veneer. It balances out the pressure on the substrate caused by the expansion and contraction of wood cells. When a veneer is bonded to both sides of the panel, each side is equally expanding and contracting and thus the panel is "balanced" - the stress of the wood cell movement is equal on both sides of the panel. This prevents warping while the veneer adhesive is curing and prevents warping caused by seasonal humidity changes in the weeks, months, and years after the project is finished. Here is a deeper explanation of both reasons for using a backer veneer.
Short Term Veneered Panel Stability
Most veneer glues contain water which causes wood cells to expand upon contact. When a panel is removed from the vacuum press (or other clamping method), the moisture from the adhesive will evaporate and the veneer will shrink slightly. By this time, the adhesive will have set its grip between the substrate and veneer and the shrinking veneer will pull on the substrate causing it to warp.
A balance or backing veneer is used on the opposite side of the substrate. It is called a "balance" veneer because it balances the pulling action of the face veneer and helps keep the substrate flat while the glue dries. It is important to allow both the face and back side veneers to dry evenly once the panel is removed from the press. See this page for details.
Long Term Veneered Panel Stability
Veneering both sides of the substrate is also important to the long term durability of the panel. Seasonal changes in humidity will cause wood cells to expand and contract. A coat or two of lacquer, varnish, or polyurethane will significantly reduce the speed of moisture change in the panel but it will not completely eliminate it. When the moisture content of the veneer changes, it will again place stress on the substrate. By balancing the panel with a veneer on the back side, those stresses are kept even and panel stays flat.
Other Things You Should Know
- A backer veneer can be applied immediately after the face veneer is pressed (but before the glue dries completely). If you work quickly, you can press the backer veneer and the face veneer at the same time.
- Almost any veneer will work as a backer veneer. The key is to use a veneer of similar thickness and grain orientation (though the latter of the two is somewhat less important). It should also be noted that the backer veneer should not be a paperbacked or 2-ply veneer unless the face veneer is also paperbacked or 2-ply. In other words, if the face veneer is a raw wood veneer, then the backer veneer should also be a raw wood veneer.
- There are some adhesives which contain little or no water. The advantage is the slight decrease in warping potential while the downside is that these adhesives are notoriously harmful to your health and the environment.
- PPR veneer glues do not dry through evaporation. Instead a PPR glue dries through a chemical process. These types of glue take 4 to 6 hours of pressing. During that drying/pressing time, the panel is kept flat by the clamping means (a vacuum press is the preferred method) so it usually comes out flat and stays flat for several days. Learn more about veneer glue by clicking here.
- Thinner substrates are more prone to severe warping than thicker substrates.
- While it's true that the thickness of the substrate has an effect on the amount of potential warping, it should also be noted that some substrate materials are less likely to warp than others. Plywood (3/4" thick) is less prone to severe warping. MDF and particle board lack the strength of plywood and may allow more warping. Learn more about substrates at this link.
- Very thick or very long substrates often can be forced to lay flat on your project framework with screws, clamps, or glue if a backer veneer is not used.
- Paperbacked and 2-ply veneers are more dimensionally stable than raw wood veneers and do not impose as much stress upon the substrate. Many cabinetmakers do not veneer both sides of a panel if they are using this type of veneer.
- If the face of your panel uses a raw wood veneer, the balance/backer veneer should also be a raw wood veneer. In other words, dont use a paperbacked or 2-ply veneer to balance a raw wood veneered panel.