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Moisture in the Woods

1/20/2014 2:23:39 PM
Article by Mac Simmons

Knowing the exact moisture content of your woods is a very important step in achieving a fine finish that will stay problem-free for many years to come.

Every woodworking and finishing workshop should include a class on “moisture in the woods,” emphasizing the importance of checking your woods with a moisture meter so you know the percentage of moisture content in your woods.

It is only by doing a moisture check, and knowing what the appropriate moisture content should be, that you can control and eliminate moisture problems like warping, cupping, shrinkage, failing joints, checking, and splitting. Once you know the allowed percentage of moisture, you also can avoid such finishing problems as blushing, pinholes, crazing, checking, adhesion, and the retarding of stains, glazes, and coatings.

Some of these moisture-related problems will show up during your finishing process. But then again, they might not show up until months after your product is delivered. This is why doing a moisture check before you start fabricating or finishing is so important, This includes all refinishers; they should always check their “stripped” woods for moisture content before finishing.

Finishing materials are not “barriers” against moisture content changes; coatings will only slow down the exchange of water molecules which exit and then return to the wood, depending on the relative humidity of its location. In many shops, the first moisture check is done when the lumber is delivered to the shop, before it is fabricated. If your lumber is not stored in a controlled environment, a second check should be done right before finishing, as the moisture content changes under different conditions. Woods that are adequately dry generally are stronger and easier to work than wet woods.

NOTE: This article will not discuss the relative humidity or equilibrium moisture content, because these values will change from location to location. However, these values are important to know. In the end, wood adjusts to its own moisture equilibrium, depending on the relative humidity of where it is stored, fabricated, finished and its final location. Your coatings and lumber suppliers can give you the correct moisture content for working your wood and for finishing in your particular location.

Using A Moisture Meter

Checking wood with a moisture meter is easy to do and will help prevent problems both in fabrication and finishing. Here is how they are used: (listing comes with most meters.)

  • Set the meter to the species of wood you will be checking.
  • Place or push the meter against or into the woods. Some meters turn on with a switch; others turn on when the pins are pushed into the woods. Wait for the value correction, and then read the final results of the meter.
  • Check a few different areas to get a better idea of overall moisture content.

A meter can pay for itself the first time it helps you find the “safety zone” for finishing and you can begin the process without worrying about eventual problems caused by moisture.

Moisture meters give valuable information on the percentage of moisture in the woods. These readings will tell you if you need to allow extra dry time or indicate that it is safe to go ahead and start your woodworking or finishing process. There are two different types: pin meters (resistance) and pin less meters (dielectric).

Pin meters have two sharp probes that are pushed into the wood. Some are pushed in parallel to the grain; on others it does not matter how the probes are positioned. Some pin meters use external cable probes that measure the difference between the core and surface moisture. Pin meters leave two holes in the woods.

Pin less meters have a flat plate that is pressed against the wood. They quickly scan large areas and do not leave any holes. Pin less meters need a smooth surface, whereas the wood’s texture or shapes do not affect pin meters.

Most meters use micro processing technology. Some display analog scales, digital LCD, and LED indicators. Certain meters can measure moisture content as low as 6% or as high as 99%. Some models use a sophisticated low frequency electromagnetic wave: others use electrical resistance.

Most meters have a setting that calibrates for corrections in the structure and density of different species of wood, while others provide a chart or table that gives instructions for calculating the moisture content. Many meters allow for “stacking” to obtain the right thickness.

Meters vary in price and size (some will fit into your shirt pocket). I suggest that every wood finisher have at least one meter in the shop. Invest in a model that fits your needs and budget. The meters are sensitive, but if you handle them with care, they will last a long time.

Take a guess; on how many wood and finishing problems are caused by moisture in the woods? Would you believe over 70% of woodworkers and finishers’ problems are moisture related?

The primary reason the problems are so high, is because so few finishers and woodworkers own a moisture meter. In addition, even when they do, they don’t use their meters on a regular basis to check wood for moisture content.

It is only by doing regular moisture checks, and knowing the true percentage of moisture in the woods; that you can try to control, or maybe eliminate moisture related problems like warping, cupping, shrinkage, failing joints, checking, and the splitting of your woods. Once you have checked your woods, and you know the allowed percentage of moisture in the woods, you can also avoid such finishing problems as adhesion, blushing, checking, crazing, pin holing, and the retarding of stains, toners, glazes, and clear and colored coatings.

Think Twice and Finish Once

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