Wood moisture and acclimation before finishing

Overview

Wood moves with its moisture content, and a finish only slows that exchange. Bob Flexner's "Finishing Twenty Questions" says that finishing the undersides of a board does not prevent warping, because a finish only slows moisture penetration. The USDA Forest Products Laboratory's Wood Handbook agrees on the principle: a film-forming finish on all sides, end grain included, will likely slow moisture change in the wood. Therefore the target is to finish wood that is already close to the moisture content (MC) it will reach in service. This entry gives that target from the Wood Handbook and says how to check it. It does not cover seasoning rough lumber or kiln schedules.

When to use

• After machining and before the first coat, on any project, especially wide panels and tabletops.
• When moving stock from a shop, mill or warehouse with a different climate from the room where the piece will live.
• Before outdoor work: see Outdoor finishing.

Materials and tools

A moisture meter, and some offcuts. The Wood Handbook describes two methods: the oven-drying method (the most universally accepted, but slow and it requires cutting the wood) and the electrical method (fast and non-destructive, but "less precise", with considerable care needed in use and interpretation).

Steps

1. Know the target. The Wood Handbook's recommended average moisture content (Table 13-2) is, for interior woodwork, flooring and furniture: 8 per cent for most areas of the United States (individual pieces 6 to 10 per cent), 6 per cent in the dry southwest (4 to 9), and 11 per cent in damp, warm coastal areas (8 to 13). For exterior siding, trim and sheathing: 12 per cent (9 to 14), 9 per cent in the dry southwest (7 to 12) and 12 per cent in coastal areas (9 to 14). It describes interior wood in ordinary buildings as typically 6 to 12 per cent.
2. Measure more than one piece. The same table notes: to obtain a realistic average, test at least 10 per cent of each item, and make several tests where the quantity is small.
3. Use the meter correctly. The Wood Handbook says to follow the manufacturer's instructions, orient and select electrodes properly, take readings after inserting the pins, apply the species correction supplied with the meter, and correct for temperature if the wood's temperature differs considerably from the calibration temperature (for conductance meters, roughly 0.5 per cent per 10 degrees Fahrenheit). Use is generally limited to values under 30 per cent, the fibre saturation point. A dielectric (pinless) meter reads mainly the surface layers, so a large moisture gradient can mislead it.
4. Acclimate. Let the stock sit in the space where it will live. The Wood Handbook's guidance is to match lumber's MC as closely as practical to the equilibrium moisture content of its service environment; the sources opened do not give a number of days.
5. Finish when readings settle, then coat all faces and edges alike; the Wood Handbook notes that a film on all sides, including end grain, slows moisture change (Finishing end grain).

Common problems

• Blistering or peeling film. The Wood Handbook warns that finishing wood with high moisture content risks blistering and peeling, and that water-soluble extractives can discolour paint.
• Warping after finishing only the show face. Flexner: finishing the underside is not what prevents warping; a finish only slows moisture exchange. Acclimation to the service environment matters more than where the finish goes.
• Meter reads inconsistently. Different species and temperature change the reading; apply the corrections above.
• Wood still moves after finishing. A finish slows, but does not stop, moisture exchange, so allow for seasonal movement in the design.

Safety and environment

No chemical hazards in this step. Conductance meters drive needle electrodes into the wood.

Related

• Finishing end grain, Outdoor finishing, Glue squeeze-out and common finish failures
• Sanding schedule, Wipe-on application
• Finishing sheet goods (plywood, MDF, MDO) for panel edges, which absorb water as end grain does