Hello
Newbie question:
I recently made a utilitarian 8 x 8 x 12 inch box out of fir planed to a half inch thick to hold a delicate piece of gear. All the joints are rabbets, and it’s glued with yellow wood glue. I put about three coats of polyurethane on it to firm up the fibers, hoping it would enable the box to better withstand some bumps and dings. The inside is unfinished.
Then I started thinking about seasonal movement. We live in Virginia which is very humid in the summer, and this box will live in a shed that has no A/C. I’m wondering how much seasonal movement of the wood can be tamed by finishing the inside of the box so that it’s completely sealed with varnish.
Does polyurethane seal the wood against humidity changes? Seems like it would, or at least shield it from wild swings in humidity. At very least, varnishing the inside can’t hurt, but I’m also ready to call it done and start something new.
Your thoughts are appreciated.
Replies
"Sealing" the wood with poly, varnish, whatever, will slow down the moisture getting into the wood, but not eliminate it. What finishing the inside WILL do is to equalize the rate of absorption between the inside and outside surfaces, and that will help prevent warping.
At 8", you won't get very much movement, but even a small amount can wreak havoc if it's only on one side of the board.
Mike Hennessy
Pittsburgh, PA
Yup varnish will only slow moisture transfer, not stop it. It fact, the product that is better for slowing water vapor penetration is shellac. (I don't recommend putting shellac over the polyurethane however.) While boxes don't really need much inside finish, especially at that size it won't hurt to put a couple of coats of shellac on the interior. Shellac dries by evaporation and hence it's odor dissipates in a mater of days. What is the delicate gear--would it be affected by the acids in most woods? If so shellac is also the best basic coating for preventing wood acid vapors from damaging delicate fabrics and other sensitive items.
Hi Steve, I hadn't thought of wood acids, though I doubt it matters for this delicate piece:http://media.rei.com/media/710730_9996Lrg.jpg... stop laughing. :-)Thanks!
No, neither do I. Where is does matter is flag cases.
I bet it does! Thanks Mike and Steve for your info. I'll hope to have a more interesting project next time.Dick.
"Where is does matter is flag cases."
Or wooden boxes for your Marcou (or even L-N) planes. ;-)
Mike HennessyPittsburgh, PA
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