plaster over ply

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I have stripped the bay of my bedroom and below the window the wall is thin ply wood - is this OK to skim over when the resto of the room is done?

Thanks
 
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It's never a very good idea to apply plaster or joint compound directly to wood.

Wood swells and shrinks with changes in it's moisture content caused by seasonal changes in humidity. The moisture content of wood always wants to reach equilibrium with the relative humidity of the surrounding air, so you can have wood shrinking indoors in the winter as it dries out, and wood swelling in the attic at the very same time as a result of humid air from the house rising up into the attic and cooling. Drywall joint compounds and thin set tiling mortars don't have the elasticity to accomodate that stretching and shrinking that occurs in wood, so if you stick them onto wood there's a good likelyhood they'll crack and peel off of it as the wood swells and shrinks.

Not to put too fine a point on it, but it's the cell walls of the wood that actually swell and shrink in thickness. Because wood cells are shaped like long thin drinking straws, there's far more cell walls as you go across the grain of the wood than along the grain. As a result, the shrinkage and swelling that occurs in wood will be much greater across the grain of the wood than along the grain. You can have up to 8 percent shrinkage across the grain of a hardwood from a saturated condition to oven dried. That's a full inch of shrinkage in a 12 inch wide board! By the same token, the shrinkage along the grain will typically only be about 0.1 percent, or about 1/80th of the shrinkage across the grain. And, that's just because of the shape of wood cells.
 
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Jbonding:

Well, yours was the shortest version, so a great big HOOAHHH! goes out to you from the US Marine Corp. ;)
 
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