The idea is that there needs to be empty space inside the wood so there is some open porosity for the sealer to go into. If the porosity is all-full-of-water, there's no room for anything more to go in there.

Wood holds water in two different ways. One is chemically-bound; this is called hydrated water because the water molecules are chemically attached to the hydroxyl groups that cellulose (wood fibers) have on them.

As the cellulose molecules take-up the water molecules, they swell in diameter. This is why wood swells when it gets wet and shrinks when it gets dry. The cellulose absorbs quite a bit of water, but only up to a limit. In this manner the wood can gain about thirty percent by weight of water. That is all the cellulose can hold in this chemically-bound manner, and that is called the fiber-saturation-point.

There is, however, empty space between and inside the cellulose fibers, and aged wood has usually experienced some degree of fungal attack; the fungi prefer to eat along the length of single fibers, since they are made of sugar molecules polymerized into cellulose, but as far as the fungi are concerned it's all sugar. As a result, some fibers are separated from their neighbors. After this kind of attack there is now more empty space between the fibers.

Water can infiltrate these empty spaces and that water is called Free water. In this manner the total moisture-content by weight of really wet wood can get up into the range of 60% to even 80% water by weight.

When the moisture content of the wood is below about 25%, you can use the sealer, but if you haven't got a moisture meter that's not going to help much.

Damp wood can be felt with the skin as a sensory organ; this is not widely known.

The inside of the forearm is for most folks very thin skin. Put your forearm against a piece of wood you know is dry and try to "feel" the moisture-content of the wood; you won't be able to feel any. In fact, it may "feel dry". Now put that same piece of skin against different pieces of wood and find the ones, perhaps down-near-the-ground, that have more moisture in them, and "feel" the difference.

This won't work on painted wood.

You can feel dry or damp soil the same way.

So, now that you know that you are a walking moisture-meter, you can judge the relative dampness of most wood yourself.

© copyright Steve Smith, 1972 - 2018, All Rights Reserved.