My brother and I built a really beautiful circular fire pit cover out of 5/4 x 6 mahogany boards. We jointed and planed the boards down to 3/4” making sure every board was perfectly flat. We used a combination of mineral spirts, linseed oil, and spar urinate for the finish. The first night we put the top on the fire pit the piece warped. Who has some advice on how to solve this issue.
it seem to be cupping upward slightly at the third board in on both sides and gets more noticeable towards the outside boards.
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First, that's not a great outdoor finish. Aside from that, wood outdoors will cup, and they always cup upward. The wider the board, the more it will cup. Six inches is a pretty fair width.
It cups upward, regardless of which way the growth rings are oriented, up or down. It's from compression of the wood cells. The tops get much wetter than the bottoms. Wet wood expands, then contracts as it dries. The cells end up permanently compressed, but just on that one side, the top.
The same number of cells, too and bottom, but the top cells are compressed, means the top is more narrow, which means it will cup. It's in the nature of wood itself.
Agree with everything John_C2 said - very tough to get exterior panels to stay flat. Other issue is that I am guessing that the panel is perhaps three feet wide and 3/4" thickness is pretty thin for that width making it even more likely to happen.
Not sure there is any "fix" now, but if you rebuild it one way to add some stability to it is to add 2-3 stiffening boards running perpendicular to the grain of the panel. Then you have to be concerned about wood expansion/contraction across the width, so the best way to deal with that is to use a sliding dovetail that is just pinned in the middle.
Good luck!
JohnC2 has it mostly right... The moisture source in this case is the earth. The wood cupping up is because the lower surface of the boards is swelling, leading to the compression on/in the upper surface that John described.
If you want to flatten it long enough to screw on braces to try and hold it flat put it in the grass during the day concave side down. The sun will dry the convex surface while the damp earth will expand the concave surface. Keep an eye on it, and when it is close to flat take your shot.
For an outdoor item like that I would not worry about movement, I'd just screw the crap out of it and let it age. If you have any of the mahogany left, cut some 1x2 strips and screw them with the 1" side in contact with the firepit cover.
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