Q. What is causing uneven stain absorption? Specifically, we can see, within each growth ring, good and sparse adsorption. This is with pine. Also, maybe it is the same thing, but why doesn’t varnish and paint stick well to pine plywood when exposed outside?
A. Good question. Indeed both questions are dealing with the same thing. So to understand what is going on, let’s go back to the basics of wood. Wood is made of small, skinny cells, which are small, hollow tubes similar to a soda straw, but only 1/4 inch long (plus or minus). However, within one annual growth ring (the rings we count to determine the age of a tree), with some species, especially pine, there are the more or less wide open cells that the tree makes during the early part of the growing season (sometimes called springwood or earlywood) and then there is a transition (abrupt or gradual depending on species) from this wood to the late season growth. The late growth, sometimes called summerwood or latewood), consists of cells that are mostly wall and have very small opening—analogy is a plugged soda straw.
So, now when we apply a stain or any sort of finish that tries to penetrate the wood cells, we find that the stain or finish goes into the earlywood cells quite deeply with not a lot of excess on the surface of the wood. On the other hand, a finish or stain on the latewood is not absorbed well, but stays mostly on the surface, giving a different appearance, especially with the first coat or two.
Note that if we were to sand the wood after the finish has dried, light sanding would first remove the finish from the latewood and then sand into the unstained, light-colored latewood. But the earlywood would sand into more stained or finished wood, as the finish would have penetrated deeply. So, after light sanding, the color contrast would be quite intense,
Now, let’s consider the question about plywood. For wood exposed outdoors, with this lack of openness in the latewood, oftentimes we find that the finish does not adhere well to the latewood itself. So when we have some movement of the wood (normal swelling when more humid or in contact with rain; or drying and shrinking from low humidity or heating with the sun, for example), the finish detaches itself easily from the wood; that is, we come to the conclusion that the finish does not stick to the wood well, as you noted.
Although this contrast between earlywood and latewood is common in most pine species, it is not common, or so troublesome, with most other commercial species. With hardwoods, however, we do have blotchiness caused by tension wood, which we have discussed before in this column.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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