By Gene Wengert, wooddoc@uwalumni.com
It is nearly impossible to detect tension wood ahead of time.
Q. I have been having a slight problem with my
oak when I finish it. It is like the oak has a little bit of fuzz on it
and so the fibers are popping up through the finish and I have to sand
between coats very carefully to get the surface smooth again. Can you
explain this?
A. Sometimes when the tree is growing, it produces a
special type of fiber that is quite weak. As these fibers seem more
common when the tree is under stress (wind, crooked stem, large
branches, etc.), the fiber is called a reaction wood or tension-wood
fiber. This fiber can have high amounts of cellulose, which absorbs
stain, glue and finishes very easily. This fiber also can be low density
and will have low strength. When the fibers, which are scattered
throughout the lumber, are machined by dull knives or dull sandpaper,
the fibers will tend to be pushed over rather than cut off cleanly. When
you put a finish on such wood, the fibers will pop up through the
finish, especially water-based finishes.
Unfortunately, it is
nearly impossible to detect tension wood ahead of time. So, your first
practical option is to use sharp tools (HSS is better than carbide in
this case) and fresh sandpaper. If the problem still persists, then you
can use a sanding sealer or glue sizing just before final sanding to
stiffen the fibers so that they will sand off cleanly.
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.
Updated: Oct 17, 2011