A butcher block maple table top required some special treatment to correct a convex downwarp that had developed near the center and ran lengthwise almost to the ends. Close inspection showed that the depression was 1/2” to 3/4” deep. The underside of the table top was bare wood that had received no finish at all. On top, the finish was clear, low luster and water resistant, probably consisting of an acrylic base.
An increase in humidity between the place the table was sold and the environment in which it was put after purchase brought about a natural change as the wood in the top attempted to acclimate itself to the new conditions. Since the uppermost surface was sealed by the finish, moisture could only enter each block from its lower surface. As each fiber in each block of the top absorbed just enough moisture to meet ambient conditions, it swelled proportionately in two directions, each perhaps an immeasurable amount, but sufficient to cause the lower surface of the table top to have greater area than the upper surface, resulting in the downwarp.
To correct the downwarp required that the moisture content of the upper and lower surfaces be equalized. Since the upper surface was sealed by the finish, it was necessary to strip the finish from this surface. This was done with a paint remover followed by a wash of lacquer thinner to remove any trace of paint remover and old finish. To further assure that the pores of the wood would be open and clean, the top was sanded with 100 grit paper and the dust removed with compressed air.
The next step was to saturate clean cloths with water which were then laid on the table in the area of the downwarp. These cloths were resaturated as needed over the next 24 hours. As the upper surface absorbed moisture from the wet cloths, the consequent expansion increased the upper area of the table top and equalized the strains that had caused the downwarp. When a straight edge laid across the table indicated that the center was just slightly higher than the edges, the wet cloths were removed and the table allowed to dry. When retested after drying it was found that the center was still slightly high. The table was then set in a sunny shop window and retested at intervals of an hour or so. Inside 3 hours the top was absolutely flat and
Preparation for finishing consisted of sanding with 100 grit paper followed by 120 and 150 grit aluminum oxide paper. The rails were sanded at the same time as were the legs. A final touch up with 150 grit paper and hand block left the surfaces clean and smoth. The underside of the table was cleaned up with 100 grit paper at this time.
Watco Danish Oil finish was used to seal the upper and the lower surfaces of the table top as well as the rails and legs. This finish is applied with brush and rag. The surface must be kept visibly wet for at least an hour, at which time the surplus is removed and the surface allowed to dry. This finish enters the pores of the first layer of wood cells where it polymerizes to form a solid. This not only seals the surface but also serves to harden it. A rub down with fine steel wool and carnauba wax completed the finish.
There is a moral in this tale. Good furniture is always sealed on all surfaces to minimize changes in moisture content.