The visible side of a veneered panel is often called the "face" side and of course it's often the most important part of the project. However, there is no point in making a beautiful veneered panel if it warps. Unfortunately, most woodworkers don't realize that both sides of a panel must be veneered in order to avoid the tragic effects of an unbalanced panel.
What is a Balance Veneer?
Any veneer used on the back side of a panel can be considered a balance veneer. If the back of the panel will show, most craftsmen use a veneer of similar color, grain, species, and thickness.
Balance-grade veneer is typically the least expensive option to use when the back side of the panel will be mostly unseen (a table top for example). Despite their low cost, balance-grade veneers are still critically important to achieving and maintaining a perfectly flat panel. Affordable sheets of balance veneer are available on the VeneerSupplies.com website.
The Importance of Using a Balance Veneer
In most parts of the world, ambient humidity fluctuates from season to season. These changes in humidity cause wood cells to expand and contract even if a finish is applied. That's how it appears at the microscopic level, but in fact, the whole veneer can slightly grow and shrink with seasonal changes in humidity. If veneer is bonded to just one side, any movement causes the panel to warp. Thinner substrates tend to suffer much more from being unbalanced than thicker ones.
A balance veneer goes on the back of the panel so that it can offset the stress on the substrate caused by the expansion and contraction of the veneer on the front side. When veneer is bonded to both sides of the project panel and each side expands/contracts similarly, the panel is said to be "balanced".
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 evaporates and the veneer contracts slightly. By this time, the adhesive has bonded the veneer well enough to the substrate that the contraction pulls on the substrate and causes it to warp.
To counteract this effect, a balance veneer is used on the opposite side of the substrate. It is called a "balance" veneer because it balances the contracting 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
The application of veneer to both sides of the substrate is also important for the long term durability of the panel. As previously mentioned, seasonal changes in humidity 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 place stress on the substrate. By balancing the panel with a veneer on the back side, those stresses are kept even and the panel will stay flat.
When to Apply a Balance Veneer
A balance veneer can be applied immediately after the face veneer comes out of the press. If you work quickly, you can press the balance veneer and the face veneer at the same time.
Other Things You Should Know
- Almost any veneer will work as a balance veneer. The key is to use a veneer of similar thickness and grain orientation. It should also be noted that the balance 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 balance 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 this type of glue cures through a chemical process. Most PPR adhesives require 4 to 6 hours of pressing. During that drying/pressing time, the panel is kept flat by the vacuum press so it usually comes out flat and stays flat for several days. However, both sides of the panel should still be veneered for long term stability. 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, but should still include a balance veneer. MDF and particle board lack the strength of plywood and may allow more warping. Learn more about substrates at this link.
- Very wide and very long substrates often can be forced to lay flat on your project framework with screws, clamps, or glue if a balance veneer is not used.
- Paper-backed and two-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, but warping can still occur under some conditions.
- Well-braced assembled speaker boxes made from rigid/stable sheet material that is 3/4" thick typically do not require a balance veneer.
Common Questions About Balance Veneer
- Do I still have to veneer the back side of my panel since it will be made from a high grade of plywood?
- Since my panel will be very large, do I still have to use a balance veneer?
- The back side of my panel will not be visible so do I have to use a balance veneer?
- I'm using a quartersawn veneer on the face of the panel so I don't need to use a balance veneer, right?
- My panel will be coated with a high-end finish on both sides so I'm not going to use a balance veneer. Will that be ok?
- If I use paper-backed veneer on 3/4" MDF, do I still have to use a balance veneer?
- I will be veneering with the iron-on method, do I still have to use a balance veneer?
- I will be veneering in a space capsule orbitting the planet. So do I need to use a balance veneer?
One Common Answer
The core part of the question really should be "Should I use balance veneer?" The answer will always be a resounding yes! You should always use a balance veneer.
The next question then becomes "Do I have to use balance veneer? The answer to this depends on your tolerance for warping. If you are using a thick substrate, the panel is small and you don't mind a bit of warping, then you might get by without a balance veneer. Is it worth the risk?
The only exception to this concern is a speaker box because it has a tendency to be thick and well-braced inside. In most cases there is no need to use a balance veneer on a speaker enclosure.
substrates can warp if you only veneer one side. The question is
how much it will warp. With the information in the chart below, you'll find that a flat cut maple veneer applied to a large, thin MDF substrate that is not braced, balanced, or cured properly would warp tremendously. A thick plywood substrate with a quartersawn walnut veneer will not warp nearly as much.
||MDF, Particle Board
|Panel Balanced Properly
|| Others (especial crotch grain)
||Flat Cut, Rotary
|Panel Cured Properly