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post #1 of 6 Old 05-15-2015, 11:40 AM Thread Starter
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Reducing wood movement with polyurethane?

I'm making some tables out of regular 2x's of Doug Fir. I am very aware of wood movement and having the proper joints to allow for the wood to move etc. What I'm wondering is, if I were to polyurethane all faces of the table top (top, bottom, sides, ends) will that reduce or eliminate any potential wood movement through the seasons? I'm wondering if doing that will "lock in" whatever moisture is in the wood and then throughout the seasons, no matter how humid or dry it is, the moisture content will remain the same or relatively close to what it was when built. Even if this will reduce wood movement by just a fraction I feel like this would be very beneficial to preventing any cracks or separations.
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post #2 of 6 Old 05-15-2015, 02:10 PM
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You can never eliminate the problems of wood movement however finishing both sides is always a good idea. It will lessen the extremes of wood movement but more important it balances one side with the other. Leaving one side raw will invite that side to swell up causing cup warp. It's kind of like wetting one side of a kitchen sponge. The thing will curl up because one side got all the water.
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post #3 of 6 Old 05-15-2015, 03:20 PM
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Every finish will still let some moisture in, the only thing it does is slow it down. So, in answer to your question, yes and no. Yes, it will keep the wood from warping too much with extreme swings, say you took it from a 10%rh environment to a 100% and back in the space of a few days. The polyurethane will slow the moisture transfer enough so you (probably) won't see any warping or checking. Since the polyurethane just slows the movement of the moisture instead of stopping it completely, the wood will still move in sustained periods of differing humidity, like through a long, dry winter

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post #4 of 6 Old 05-15-2015, 03:31 PM Thread Starter
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Got it....I'm going to start using the poly all around the tops to help with this. To start off though I already use KD wood, so my moisture content is already at 7%-8%. My tables will also be in a climate controlled home.
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post #5 of 6 Old 05-15-2015, 07:55 PM
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One thing to make sure of, if you haven't already, is let the wood acclimate to the new environment before you start working with it. If the relative humidity at the store is 20% and your shop is 10% and you put your project together the same day you bring the wood home, well, it'll probably turn into a potatoes chip. Personally, I like letting mine sit for a couple days, but your milage may vary

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post #6 of 6 Old 05-16-2015, 07:41 AM
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7%-8% moisture content seems low for framing lumber. I thought KD framing lumber was generally in the 15% range.
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