Nail Lifting Problem on Finished Plywood Bar

      A discussion of why plywood moisture changes may cause pin nails to lift out in service. April 24, 2009

I built a simple bar for a café , two units, each 4’x6’x2’. The frame was #1 pine, the front and sides sheathed with ½” A/C pine ply and the top with 5/8” A/C pine ply. The ply was glued and nailed with 15ga. nails and the holes were filled with Famowood. It was then primed and a black polyurethane ‘piano finish’ was applied.

This was in April before our rainy season. Now the nails on the top have lifted very slightly. You can barely feel them but with the brilliant finish, they stand out. The nails on the front and sides are not doing this. There is also a slight bulge of the top around the sides. The butt joints on the corners with the ½” are also okay. I told the client that it was a humidity problem and let’s wait and see what happens after our rain season. Perhaps if it’s seasonal she’ll give me a pass on it.

Unfortunately, here in Mexico, we cannot get birch ply; the A/C five-plies core ply is the standard. ¾” ply is limited to five plies, even if the faces are oak or maple. I’m assuming my problem is because of a ‘green’ sheet of the 5/8” ply, as the ½” is okay. It would almost appear that two things are going on: it's swelling in both directions longitudinally and shrinking in thickness.

What are some possible solutions if I have to repair this? The thought of breaking it down and moving it back to my shop, replacing the top and refinishing is daunting. But I may have to.

Forum Responses
(Architectural Woodworking Forum)
From Gene Wengert, forum technical advisor:
The problem does indeed sound like an increase in humidity that then pulled the nails out just a bit. The wood then shrank a little and the nails were left "pulled out" a little bit. If you cannot avoid exposure to high RH, then this is likely going to be tough to fix, as every year it will occur again. At this point you can renail (drive the nails in) and then fill and refinish. The bulge may not be possible to repair. I certainly wonder if these pieces had a moment of exposure to very high RH or even liquid water.

From contributor A:
Wait and see what happens is the best bet. I think it's possible that your ply has actually shrunk slightly (the cafe could be drier than the timber yard or your workshop) and the nails have remained in their original position. Either way, only major refinishing will eradicate it, possibly not permanently.

From contributor G:
I believe the nails will continue to push out. As a framer I've seen large nails pushed completely out within a few months. Actually quite common and is why various nails have been invented with designs to help prevent. Screws work best, "screw and glue".

From Gene Wengert, forum technical advisor:
Incidentally, ring shank nails would be a good idea.

From contributor U:
The problem here is the inadequacy of nails in this situation. The mechanical attachment is of primary importance and should never be left to such rudimentary connection devices, especially when buried under the finish as you describe. Using a good quality screw would most likely solve the problem, the wire nature (smooth shaft, glued or not) of the nails means that even if the relative humidity remained stable, the expansion/contraction due to temperature differences alone would walk the nails out of their positions.

Ringshank is a better choice, but it goes partway to the total solution of a decent wood screw. Look into stainless steel if you never want to revisit the screw again. Galvanized at the very least, but I don't have as much experience with those. You'll probably need to rework the board, rework the attachment devices and refinish the project. You know your materials better, you might rework the warranty a little as well to accommodate all the situations you have no control over. Ideally, the warranty should protect the buyer from shoddy workmanship and protect the manufacturer from shoddy application or maintenance.

From Gene Wengert, forum technical advisor:
To clarify a bit: Wood shrinks and swells with moisture changes, but does not change size with temperature if the moisture stays constant. Nails change size with temperature, but not with changes in moisture or humidity. The withdrawal resistance of a smooth wire nail is proportional to the diameter and the length of penetration.

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