Should you seal both sides of plywood?

Not at all. In fact, all the evidence points the other way – that it doesn't make any difference whether the wood is finished on both sides or not. It will warp or not warp totally independent of how it's finished, because a finish only slows moisture exchange.

popularwoodworking.com - Finish Both Sides? Not Necessary. - Popular Woodworking

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Logic is usually a good guide, but it doesn’t always apply. A case in point is the widely held belief among woodworkers that the way to prevent warping is to apply finish to both sides of a board.

This practice does seem to make sense at the outset. Moisture exchange is responsible for warping, and a finish slows moisture exchange. So assuming the wood was properly kiln-dried to begin with, if the moisture leaving or entering the wood from changing humidity conditions is equalized on both sides, wouldn’t this prevent warping?

Not at all.

In fact, all the evidence points the other way – that it doesn’t make any difference whether the wood is finished on both sides or not. It will warp or not warp totally independent of how it’s finished, because a finish only slows moisture exchange. It doesn’t stop it. Both sides of the board will adjust reasonably rapidly whether they’re finished or not. (This is not to say that you shouldn’t finish both sides, especially if the underside or inside will be seen or touched. A finished surface looks and feels nicer to most people than an unfinished surface.) Let’s look at two objects you’re very familiar with (tabletops and decks) to explain what actually causes warping, then use the explanation to understand how to correct warps when they do occur.

Warped Tabletops

Have you ever noticed that old tabletops, especially drop leaves, always seem to cup concave on the top? You rarely, if ever, see a tabletop bowed convex on the top. Usually, these old tops are unfinished on the bottom, so you may have assumed that this accounts for the cupping. But it doesn’t. In fact, if the unfinished side made a difference, it should have caused the top to warp in the other direction – convex. The bottom would have dried out and shrunk faster than the top as interior conditions in buildings became increasingly drier over the last century. Nor does the natural shrinkage of plain-sawn boards (twice as much around its rings than perpendicular to its rings) explain the concave warp. Old plain-sawn tops almost always have the heart-side up, and this should mean that any shrinkage occurring because of the furniture adjusting to drier interior climates would result in the top bowing rather than cupping. With regard to quartersawn tops, the natural shrinkage occurs fairly evenly in both dimensions, so there is very little or no warping because of drier conditions and shrinkage. Yet quartersawn tabletops sometimes cup just as much as plain-sawn tops.

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Warped Decks

Take another example: Boards placed in a deck almost always cup on the top despite usually being laid randomly. Some boards have their heart-side up, some have their sap-side up and some are quartersawn. The concave warping occurs whether or not the boards are painted, coated on the top with a water repellent or deck stain, coated on both sides, left totally unfinished or if they are in the sun or shade. When the cupping stresses become great enough, the boards check and split. There must be an explanation for warping other than the ring pattern of the wood or whether the wood is finished on both sides. And there is. It has to do with the greater amount of moisture that comes in contact with the top of a tabletop or the top of a deck.

Compression Shrinkage

Over a period of many years, a tabletop can be wiped thousands of times with a damp cloth. As the finish ages, it becomes more porous and lets moisture through, so the wetness gets into the wood. Likewise on a deck, more moisture enters the top surface than the bottom because rain wets the top more than the bottom. When moisture enters wood, it causes the wood to swell. The top surfaces of the tabletop and deck thus try to expand. But the wood’s thickness remains stable and prevents this. As a result, the cells at the top surface are compressed from their original cylindrical shape to an oval shape. When the wood eventually dries out again, the cells don’t return fully to their cylindrical shape. The top surface thus shrinks, pulling the board concave. Each time the top is wetted and dries out, it shrinks a little more. This phenomenon is called “compression shrinkage” (or “compression set,”) and it explains warpage and eventual splitting when neither the ring pattern of the wood nor a finish applied to one or both sides does. A built-up film finish in good shape resists water penetration pretty well, but a deteriorated finish doesn’t. So refinishing whenever a finish ceases to serve its protective function extends the useful life of the object. This is the problem with the “do not refinish” message being conveyed by the popular television series, “Antiques Roadshow.” If people heed this message, a lot of furniture will be destroyed over the long term. Thoughtful refinishing can extend antiques’ lives.

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Straightening Warps

Understanding warping caused by compression shrinkage helps us find a method for straightening warps. Recreate on the bottom the same conditions that caused the cupping on the top. You could do this by wetting the bottom and letting it dry out many times until the wood flattens out, but there is a faster way. Place the warped board upside down and hold it firmly in clamps so it can’t expand. Then repeatedly wet the upper bowed side by covering the board with a wet cloth. At the same time, place some weight on this side to encourage compression shrinkage. Once thoroughly wet, remove the wet cloth and let the wood dry, with the weight still in place. You can encourage the flattening even more by introducing steam using wet cloths and a hot iron. You have to be very careful when doing this not to put so much pressure on the wood with clamps or weight that you cause it to split, or that you soak the wood so much that you cause glue bonds to separate. If there are severe splits in the wood, this fix probably won’t work. As few as two cycles of wetting and drying should result in some improvement. Usually it takes quite a few cycles to bring a severe warp reasonably flat again. However many wettings it takes, the real benefit of this type of repair is that it does no damage because you’re always working on the bottom, unfinished side. PW

popularwoodworking.com - Finish Both Sides? Not Necessary. - Popular Woodworking

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