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Old 10-31-2008, 04:41 PM   #16
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Old 10-31-2008, 11:31 PM   #17
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Originally Posted by n0c7 View Post
Does DAP need to be primed before painted? I used alot of it during my last project on the baseboards(white baseboards, white DAP). Now that everything is settling I'm starting to see alot of paint cracking in the mitre joints and other spots that were caulked. I didn't prime these spots. Trying to determine cause before attempting to simply paint over again.

I don't think the gaps in the mitered joints have anything to do with the caulk you used or whether or not you primed it. I suspect the gaps are merely the result of the wood drying out and shrinking slightly as it does.

Normally, wood is dried well before it is cut into moldings, but it may have absorbed humidity from the air if it was stored in a humid area before being installed. Also, it may simply be drying out because of the seasonal change in humidity.

It's now the end of October. If you installed this wood trim in the summer, then the indoor humidity during July (say) would have been much higher than it is now. Now, at the end of October, you have cool temperatures outside, causing the outdoor air to be drier to begin with. That outdoor air gets inside our house and gets warmed up, causing the relative humidity of the air inside your house to be very low. Wood continuously exchanges moisture with the surrounding air to maintain an equilibrium moisture content with it. So, lower indoor relative humidity means the wood is going to lose moisture to maintain moisture equilibrium with the surrounding indoor air, and that spells "wood shrinkage".

Wood cells are shaped like long tapered drinking straws, closed off at both ends. When wood dries, initially the water INSIDE the cells evaporates. When that happens, the only change in the wood is that it gets lighter in weight. Once the water inside the cells has completely evaporated, then the water in the cell walls starts to evaporate, and the cell walls shrink in thickness and become stiffer. The increased stiffness of the wood cell walls is why dried wood is stronger than wet wood. The point at which the cells are just empty of water, but no water has evaporated from the cell walls yet is called the "fiber saturation point" condition. Typically, from a "saturated" condition to oven dried (which is zero moisture content), wood can shrink up to 11 percent across it's grain. That's more than a full inch of thickness in a 2X12 ! But, since it's only the cell walls that shrink in thickness, the shrinkage ALONG the grain is typically very much less, being only about 0.1 percent. It's the very much larger number of cell walls you encounter as you travel across the grain that results in the much higher shrinkage across the grain than along it.

(If you consider the trunk of a tree as a collection of nested cylinders, with each cylinder being a tree ring, or year of growth, shrinkage as wood dries is greatest in the direction tangential to the tree rings. It's about half that much radially through the tree rings and considerably less vertically along the wood grain.)

I'm thinking that the cause of the shrinkage may simply be the lower relative humidity inside your house lately, over the past 2 or 3 months, say. You have cool dry air outdoors, and that air gets in your house and warms up, causing it's relative humidity to drop way down low. Your wood trim is going to lose moisture to the surrounding air to maintain a moisture equilibrium with it, and that means your wood is going to shrink across it's grain a lot, but along it's grain a bit as well. Hence, the tiny gaps that form in the corners.

My feeling is that you'd have encountered exactly the same problem regardless of who's caulk you used and regardless of whether or not you'd primed it.

If wood shrinkage is the culprit, then you should see those gaps largely disappear by next July when the relative humidity in your indoor air is back to what it was last July.

I agree that the fix is to caulk those gaps and paint them. As the wood expands next July, it'll squeeze that caulk a bit. But, wood is a soft material, and so is latex caulk, so there'll be a bit of compression of both at the corners, but that won't do either the wood or the caulk any harm.




Last edited by Nestor_Kelebay; 11-01-2008 at 12:16 AM.
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