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Question:
I have installed a 8" pine log sliding 1" thick in the middle with a 1/2" overlap. I used Cetol 1 and put 2 coats on it the first year. I was planning on putting on a third coat last year when I noticed some black spotting ocurring. It would appear most of the spotting is ocurring half way down from the top and arround decks. This pine was kiln dried prior to us milling it into sliding. It also sat in my garage for 4-6 months prior to installing it on our cottage. I have pictures I could email y
Answer:
All wood contains polyphenolic compounds called tannins.
Tannins are contained within wood's cellular structure and are
distributed throughout the
wood. However, if the
chemistry of seasoned wood
i s d i s r u p t e d , h i g h
concentrations of these
tannins can migrate to the
surface and, once there, react
with minute metallic particles of iron that remain on the surface from
the hewing, sawing or planning process. The resulting iron tannates
form dark colored discolorations that can appear as streaking spots
or large dark blotches, sometimes covering an entire wall. This
process often takes some time to occur resulting in tannate
discolorations showing up several months after a finish system has
been applied.
Bleach residue is responsible for most of the problems related to iron tannates but any product with a high
pH will do the same thing. That's why thoroughly rinsing any cleaner off the surface of the wood before
staining is so important. If iron tannate stains are present on bare wood, they can be removed with a
solution of OXcon oxalic acid. However, if they are under a finish, the only way to remove them is to
strip off the finish and then use OXcon.
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