In Getting Started in Woodworking (2003), Aimé Ontario Fraser wrote that sheet goods such as plywood and MDF are “uniformly flat, dimensionally stable, easy to machine, and virtually unaffected by changes in relative humidity.”
Does this apply to all grades of plywood?–because I just bought some plywood and was surprised to find that it has a slight bow. The face of the board has very slight waves as well. I didn’t specify what grade; just asked for “1/4-inch birch plywood” and they gave me the first sheet in the stack.
Janet
Replies
Many sheets of plywood and even MDF will take a bow if stored that way for a period of time. I wouldn't be surprised even if a sheet always stored flat wants to bow - not usually a temporary condition due to moisture.
Waves would be due to poor quality control during manufacturing. May be an overlap where inner ply ended and another started, or uneven glue application. Can't sand flat without next layer telegraphing through or sanding through the top layer entirely.
"Does this apply to all grades of plywood?"
No. Most (all?) of the plywood found at home improvement stores is imported.Often, it's almost dripping wet when sold. That's OK for some carpentry applications, like concrete forms, and such, but not for furniture or case work. My suggestion would be to find a lumber store that stocks, or can special order for you, domestic furniture-grade plywood. You should expect significantly higher prices, though.
Such Things Are Stable
Such things are stable in that they do not change dimensions with changes in RH. While there are caveats to that statement, it is a general axiom that one can design and build with. However, all wood products respond to changes in environmental moisture since they can still absorb and give off moisture.
Plywoods stored flat will expose one side of the top sheet to the environment, and that one side may bow up in the center if the humidty is rising, or the ends may curl up if it is dropping. A sort of hygrometer.
Years ago, when observing stacks of white pine frame and panel doors, the top door would bow up when the humidty was rising. These doors were judged to be 'bowed' by the door hangers, and set aside. Later, we would notice the doors were not bowed, and they could go back into production.
Sometimes the doors curled up at the ends - when humidty was dropping. Set aside again, only to find they 'fixed themselves' later.
Once I realized I was just seeing a response to humdity changes to one side of the door, we started using cover sheets of 1/2" MDF. All the doors were always flat coming out from under the MDF cover sheets, and the reject rate droped to nil. Indeed the manufacturers of these doors - having stored them flat over their 60 yerars of production, use cover sheets to help keep everything stable.
Curiously, when looking at the mass produced door industry today, the newer, less experienced segments of the industry do not know this simple fact, and cite the use of particle board - or worse - stiles and rails as a response 'solid wood warping'. The irony is that - you guessed it - the mass produced particle board doors still will bow with rising humidty since they are wood products. With the lack of experience and knowledge, the makers just set the problem aside as if it is unsolvable, and yet another 'mystery' of wood.
Dave S
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