painting plywood


Most of my woodworking has been with solid wood, and I almost always apply the same type and number of coats of finish to hidden and show surfaces to assure balanced moisture movement.
Now I am building a bookcase/cabinet built-in that will be painted. And I suddenly had to ask myself why I was planning to paint surfaces that will never be seen. Is there any reason to? e.g., the back side of the back panel or the underside of the cabinet floor. If humidity changes flow mainly through those surfaces and not the painted "show" surface, does it matter with plywood? This will be in an air conditioned space, and while it is on an outside wall our temperatures (in Atlanta) don't get as extreme as some areas.
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Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

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alexy wrote: > Most of my woodworking has been with solid wood, and I almost always > apply the same type and number of coats of finish to hidden and show > surfaces to assure balanced moisture movement. > > Now I am building a bookcase/cabinet built-in that will be painted. > And I suddenly had to ask myself why I was planning to paint surfaces > that will never be seen. Is there any reason to?
Why not hit the hidden surfaces with a coat of shellac?
Lew
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Little reason not to (small cost in $ and time), and I am a big fan of shellac. But is there a reason _TO_ hit is with a coat of shellac??
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alexy wrote:

I hope not, the built-in I recently did was all plywood and only painted on the inside. It has been a few months and no problems yet even with all of the heat and humidity in the garage and the AC running full blast in the house. If it matters the back side is fully insulated and drywalled on the garage side.
If you're looking for some ideas this is where I got my inspiration: http://www.finewoodworkers.com/detail_pages/t_builtin.htm You can click on the pictures to get a better view.
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alexy wrote:
> Little reason not to (small cost in $ and time), and I am a big fan of > shellac. But is there a reason _TO_ hit is with a coat of shellac?? >
Other than to seal and protect the wood, no reason.
Lew
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The conventional wisdom is that if you don't treat both surfaces the same they can move differently and warp. Things that are held securely, like cabinet floors, probably are going anywhere. And I suspect in climate controlled houses there isn't much effect anyhow. Still, I do both sides if it is practical. The time and material is not large, and it might prevent a problem.
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