It’s a widespread myth among woodworkers and finishers that to prevent warping it’s necessary to balance moisture-vapor exchange by finishing both sides of wood.
In fact, finishing the undersides of tabletops or the insides of cabinets or chests has only limited impact on reducing the likelihood of future problems. The only reasons to go to the trouble are for looks and feel — both of which are perfectly legitimate. But neither has anything to do with stabilizing the wood.
Causes of warps
There are three causes of warping. The first and most important is inadequate kiln drying. Unless the moisture content of the wood is brought into equilibrium with the average humidity conditions of the location in which the wood is to be used, it will continue to adjust.
If the wood is quartersawn, it will shrink across the grain. If it is plainsawn, it will also warp (cup) on the sap side as the longer, outer sap-side rings shrink more than the shorter, inner heart-side rings.
The second, and less common, cause is the wood (furniture, etc.) being moved from a very damp to a very dry climate, or visa versa. The wood adjusts to the new humidity conditions and may cup if it is plainsawn.
The third, and still less common, cause is repeated exposure to water on the topside leading to compression shrinkage and cupping — no matter what the annular ring orientation.
In none of these cases does finishing the undersides or insides provide any significant resistance to warping.
While still part of a tree, wood contains a lot of water. Most of this water has to be dried out before the wood becomes useful for furniture or cabinets.
In the old days, this drying was done by stacking (and “stickering”) the wood under cover until it reached a moisture content in equilibrium with the moisture in the surrounding air. Because buildings were not tight and heating was poor, the wood remained fairly stable.
To speed the drying process, we now remove most of the water in kilns, but the amount removed is often inadequate or the dried wood is then stored in damp conditions where it can take on moisture. When this wood then adjusts to the drier conditions in modern centrally heated buildings, it shrinks, and it warps unless it is quartersawn.
Finishes can slow the drying of the wood, but they can’t stop. All finishes allow moisture to pass through in vapor form. You experience the inadequacy of finishes (and paints) to fully encase the moisture in exterior wood doors that swell and stick in the summer and shrink and allow air through in the winter, even though the wood is finished (or painted) on all sides.
Some finishes slow moisture exchange better than others, and all finishes do this better the thicker they are. For example, coating wood on all sides with 1/8" thick epoxy resin comes pretty close to completely stopping any moisture exchange.
But we don’t use thick epoxy-resin coatings on cabinets or most furniture, and we don’t build our finishes to 1/8" thick. So, we can’t expect the finish to completely stop moisture exchange. Inadequately dried boards will still adjust and warp even if they are totally encased with finish on all sides. They just take a little longer to do this.
Moving furniture and cabinets from damp to dry conditions or visa versa leads to plainsawn boards warping just like beginning with boards that are too damp or too dry. Of course, moving from dry to damp will cause the board to swell and cup on the heart side.
This warping will occur even if the wood is finished on all sides. It will just be slowed a little as the moisture takes longer to get through the finish.
The least understood of the three causes of warping is compression shrinkage. This is a phenomenon that occurs usually on the tops of flat surfaces such as tables, decks, cedar shakes, floorboards and cutting boards.
Compression shrinkage (or “compression set”) is a technical term used by wood technologists to describe a condition in which the cylindrical cells of cellulose in wood are not allowed to expand when moisture is absorbed, so they get compressed into oval shapes. Compression shrinkage explains how screws work loose in wood, and wooden handles become loose in hammers and hatchets. It also explains splits in the ends of boards and checks in the middle of boards in addition to warping.
When water enters wood, the cellulose cell walls swell. If the metal head of a hammer or hatchet doesn’t allow expansion, or if the wetting is only on one side and the thickness of the wood prevents the cells from expanding, they become compressed into oval shapes. When the wood dries, the cells don’t resume their cylindrical shapes and the handle, or the wetted side, shrinks a little.
Each time the handle or the one side is wetted and dries out, it shrinks a little more. Repeated wetting and drying eventually leads to the handle working loose or the one side cupping, and if the cycle continues long enough, the wood splits and checks.
Tabletops come in contact with water when being wiped down with a damp cloth after meals. Decks and shakes are exposed to water on the topside every time it rains. Floorboards are exposed to water when damp-mopped. Cutting boards are wetted during meal preparation and cleanup.
When the finish on tabletops and floorboards is too thin or becomes worn or crazed and doesn’t adequately prevent water penetration, the cells near the surface go through compression shrinkage and the wood eventually cups.
Clearly, finishing tabletops, deck boards, cedar shakes, floorboards or cutting boards on the bottom sides won’t have any effect because the cupping is being caused entirely by what is happening to the topside.
Where did it start?
If it’s so wrong that finishing undersides and insides prevents warping, how did this myth get started?
Consider that until the 1920s and ’30s and the introduction of sprayed lacquer, both sides were rarely finished. If it were important to do so, you’d think furniture makers over the previous two or three centuries would have figured it out.
Also consider that none of the dozens of old finishing and refinishing books I’ve looked through, including many targeted at the furniture industry, even mention a need to finish both sides.
The first mention I was able to find was in the third issue (summer 1976) of Fine Woodworking magazine (page 6). Even here, the instruction was only glancing. “I always finish top and bottom equally, which helps prevent warpage.”
In the next issue, wood technologist Bruce Hoadley recommends “proper finishing” to reduce or eliminate wood “instability” (page 24). But proper finishing wasn’t explained.
By 1980, in his book “Understanding Wood,” Hoadley was clearer (page 189). “It is crucial that all sides of every board receive equal finish. The concept of balanced construction also applies to finishes.”
From this point on, the instruction to finish both sides took on a life of its own. It appeared often in all woodworking magazines and in many books — always presented as a statement of fact, never questioned and never accompanied by any evidence that it made any difference.
I believe woodworkers are susceptible to the idea of finishing both sides because they know veneering only one side definitely leads to warping. They know they must apply veneer to both sides of a panel or it will cup on the single veneered side.
But the two situations are entirely different.
Cupping occurs on single-sided veneered panels because of the use of a water-based adhesive. The water swells the veneer as it is being laid. When the veneer then dries out, it shrinks, drawing the panel into a mildly cupped warp. There is no cupping when solvent-based contact cement is used to attach veneer to just one side.
Finishes don’t shrink with enough force to cup a 3/4" panel. So there’s no stability justification for finishing both sides — just looks and feel.
Bob Flexner will present a two-hour seminar, “Cabinet Grade Finishing for the Small Shop,” at IWF2008 in Atlanta on Friday afternoon, Aug. 22.