View Full Version : cupping wood

james burt
06-15-2005, 09:36 AM
i am testing out a piece of wood to see if i like
the finish and how well it will hold up exposed to
the weather.
i am pretty sure it is pine, bought from the local
big orange. it was dry. i put 3 coats of a teak
oil finish sealer. formby's, i think.
let it dry between coats for a day or so.
looked nice while it was in my shop. left it outside, has been raining like crazy here for the
last 2 weeks. noticed this am, that it is cupped
bad enough that when turned over it will rock.
not being a wood person, what did i do wrong? :confused:



Bob Smalser
06-15-2005, 09:48 AM

In wood, especially flatsawn wood, every coat of finish that goes on the top also has to go on the bottom, or moisture enters the bare side and swells it without swelling the coated side.

Bruce Hooke
06-15-2005, 10:57 AM
Also, the fastest way to get a board to cup is to put it down on a damp surface, like the ground, and leave it there, especially if the sun can shine on the other surface. This is pretty much a sure-fire way to cup a board in short order. If you board was up off the ground and out of the sun then the above clearly would not apply.

Basically, rapid cupping is caused by one side getting wetter than the other. This can happen because one side is finished and the other is not, but the most common reason in my experience is because the board was placed such that one side was exposed to a lot of moisture and/or one side was exposed to a lot of solar drying. A board can also cup as a result of the moisture content of the whole board going up or down, but this generally takes more than a week or two to show up (but if the board was allowed to get really wet then that could be what is going on, or part of what is going on).

For what it's worth, lumberyard pine will not be very durable in a marine setting...

06-15-2005, 12:00 PM
Though SYP (southern yellow pine) is pd durable (and available at HD, at least in these parts).

james burt
06-15-2005, 12:26 PM
forgot to mention that all sides were equally
treated, 3 coats. but with it being pine and
yes, it was outside, in the rain, been wet for
2 weeks straight and one side was down on a
railroad tie. so i guess with all this against it
no reason it cupped so fast.
guess i need to order some mahogany and try it out.
as always, thanks for everyone's help. i might
even be able to cut a piece of wood (straight)
by the time i'm ready to build one.
any suggestions for a large piece of cupped pine? smile.gif

06-15-2005, 02:38 PM
any suggestions for a large piece of cupped pine? Actually, yes. Lay the board convex side up on the grass in the sun. Leave it there for a while until the board cups the other way, thereby flattening itself out. If you let it go too far and you reverse the cup, just flip it over and monitor it more closely.

The effect can be accelerated by wetting the concave surface.

Once is reasonbly flat, move it out of the sun and store it on edge in your shop. Use it as needed.

Bruce Hooke
06-15-2005, 02:44 PM
Almost any wood will cup if left sitting on a wet surface, like a railroad tie in the rain, so all you have really "proved" is that you were testing a piece of wood! :D

n.b., but some woods will certainly cup more than others...

06-15-2005, 02:59 PM
Study the grain lines on the end of the board, the more that they parallel the width of the board the more likely the board will cup, the more nearly the growth lines parallel the edge, the less likely the board will cup. But any board exposed to the weather and the sun will start to curl and the longer it is allowed to weather the worse it will get.

Bruce Hooke
06-15-2005, 04:01 PM
Originally posted by ssor:
...any board exposed to the weather and the sun will start to curl and the longer it is allowed to weather the worse it will get.I would modify this slightly to say that any board that is exposed in such a way that one side stays wet and the other side is much drier will cup. A board that is pretty much equally exposed to moisture and drying on both sides may not necessarily cup.

Part of what is confusing things here is that we potentially have two factors going on:

1. Short term -- if one side of a board gets wet and the other side does not, then pretty much any board will cup, and do so very quickly (lay a board down on the lawn in the sun and it will cup in a matter of an hour or two). A flat-sawn board will cup a bit more than a quartersawn board, and a board made of wood that moves a lot when exposed to moisture will cup more than a more stable wood, but except for really oddball woods that do not react much to water, they will all cup. However, if you remove whatever is causing the board to stay much wetter on one side than the other then it will flatten out again as the moisture content equalizes throughout the board. This whole process is being driven by uneven moisture content between the two sides of the board.

2. Long term -- if you substantially change the overall moisture content of a flat-sawn board (the growth rings are parallel to the flat face of the board when you look at the end of the board) it will warp. However, a quartersawn board (growth rings are perpendicular to the flat face of the board) generally will not. The reason for this is more complex -- it has to do with the fact that wood shrinks and swells more in some orientations relative to the grain then in others. If you return the board to it's original moisture content it should flatten out. This is the more usual reason for cup, because over time unless one side of the board is kept much wetter than the other, the moisture content in the board will equalize, thus negating effect #1.

In the situation presented by James, I suspect we are mostly dealing with #1, but if the board was out for a couple of weeks then #2 could also be coming into play.

06-15-2005, 04:14 PM
The other factor over which we have no control is reaction wood stresses. Where-in a tree has grown leaning to one side and is sawn into straight boards. When we cut them they can pinch the saw or spring open starting at the beginning of the cut.
I once saw a 2x4 by about eighteen feet that was all bent and twisted and leaning against the outside of a local lumber shed. I asked Lester about it and he said that he found it about 2 months before bad enough to not be sellable and wanted to see how far it would go. It had at least a four foot bow in it and wouldn't lay flat no matter how it was laid on the parking lot.

06-15-2005, 04:20 PM
It is my impression that kiln dried wood, often sap wood, not quarter sawn, will bend and curve all to hell in the elements. I used to search for planks in mangrove swamps, occasionally finding weathered planks of pine or spruce (or god knows what tropical woods) that had been cast up into the trees and roots by hurricanes. These planks didn't distort when wet, hadn't rotted after a few years in the mangroves, were fine boat building wood. I have a couple friends who used a good bit of such "drift" wood in the construction of various boats and even a small house. Of course a deck load of 3/4 inch exterior grade fir plywood that drifted up into the mangroves didn't go to waste either.

[ 06-15-2005, 05:21 PM: Message edited by: Tristan ]