Greetings,
I have been doing some research on veneering. I noticed that if you use plywood you need to veneer the underside, too. Why is this? If you are only adding another “layer” to the plywood, why veneer the underside? So, if I use MDF, do I need to veneer the underside? I’m sure someone here can explain this to me.
Thanks,
Joe
Is it really saw dust or wood dust?
Replies
It's done to keep the equilibrium of the sheet. Plywood is always an odd number of plies (5-ply, 7-ply, etc) and if you add a layer on one side you should add a comparable layer on the other. MDF is less prone to destabilize, but it is a sensible practice to also glue up a "equalizing" layer of whatever you're glueing - cheap veneer, laminate, etc.
DR
Thanks Ring. I'll experiment with both.Joe
Is it really saw dust or wood dust?
>> Plywood is always an odd number of plies (5-ply, 7-ply, etc)
Not true, I've used plenty of plywood, that was even layered. Very common in construction grades.
I agree with the need to veneer both faces for equilibrium, but that can be achieved whether you have an even or odd number of layers. The more important point is that the grain in both outside layers runs in the same direction (for more than one reason).
Sharp,
I've never seen even-number ply. It means that the grain will be running lengthwise on one face, and crosswise on the opposite face. Maybe it exists for some construction purposes, but I've never come across it for any furniture related use.
DR
note - if you think about what you said regarding outside grain direction, you'll see that it must be odd-number of plies.
Edited 12/15/2005 2:39 am ET by ring
>> I've never seen even-number ply.
OK, I believe you. That doesn't mean it doesn't exist. All I was doing is refuting the original statement that ply does NOT come in even numbered layers, it does. There was no distinction made for what the ply is used for.
I know this forum is woodworking in the context of furniture making, just wanted people to be aware that a lot more plywood (different types) is used in construction than in furniture making.
>> if you think about what you said regarding outside grain direction, you'll see that it must be odd-number of plies.
Not true, flawed logic :-) as others already mentioned, the inner/middle plies run in the same direction.
The construction grade plywood with an even number of plys have the grain in two of the inner adjacent layers running in the same direction, so that a four ply sheet is essentially a three ply. This is the only way to get the two face plys running in the same direction. This is acceptable, apparently, for construction grade stuff but I've never seen this done with better quality plywoods, they always have an odd number of plys.
John W.
JohnWW:I think plywood for tongue and groove floor underlayment has an even number of plies that way, so that the tongues will have the grain of the two center plies for strength.
Seems like a reasonable idea, I'll have to look at a sheet of T&G ply.
John W.
I don't dispute that even plied plywood exists although I have never seen or heard of it. I worked in a plywood mill for a number of years and I can tell you some reasons that would make it less practical. Number one is the cost of resin (glue): for each additional layer it adds to the cost of the panel. Number two is the cost of labor: each layer of veneer that has to be laid adds to the cost of the panel. Number three is it's cheaper to turn a thick veneer on the lathe than twp thin ones. The only thing that comes to mind as far as a reason is that it is difficult to turn thick veneer from skinny logs.
BTW the core (cross plies) aren't a solid sheet, they are made up of narrow peices laid up. That's where gaps and overlaps come from - improperly or carelessly laid core.
Edited 12/16/2005 2:26 am by polarsea1
>>> so that a four ply sheet is essentially a three ply.
I see, 4 = 3 ?
No issue with the rest of your reply :-)
Yes, a four ply sheet is more like a three ply with a thick middle ply. I suppose that by using two layers in the middle instead of one thick ply there is less chance for large flaws in the middle layer from weakening the sheet and they can use the same thickness plys for all layers.
John W.
Joe,
MDF expands and contracts with changes in humidity. If only one face of the MDF is covered with a veneer, then that face will be restricted from moving and will also absorb or lose moisture at a different rate than the unveneered face, which will cause the panel to cup.
John W.
Thanks JWW, I didn't think of that.Joe
Is it really saw dust or wood dust?
John,But what if you seal/finish the backside of the MDF? Would that preclude equilibrium problems?********************************************************
"It is what we learn after we think we know it all, that counts."
John Wooden 1910-
Nikki,
The basic idea is to equalize (more or less) the forces acting on both sides of a sheet of ply, MDF, whatever. The simplest and surest way to do it is to veneer a cheap veneer on the reverse side, and to finish it with the same products, although obviously it can be done with less care than the good side. Trying to figure what finish might correctly counterbalance the good veneer is an exercise in wood engineering that very few of us are capable of. A cheap layer of veneer is just that. When the solution is simple, obvious, and foolproof, I don't see the logic in trying to outsmart myself.
DR
Basically no, for two reasons:
While the backside finish would change the rate at which moisture diffused into and out of the board, the rate would probably not match the moisture gain and loss through the veneered face, so the front and back of the MDF would still move at different rates.
Second, and this is actually the primary reason it wouldn't work, is that the veneer glued on the front will restrict the expansion and contraction of the front face of the MDF, especially in the direction of the veneer's grain which doesn't move with moisture swings.
In fact, the best practice is to put two layers of veneer, laid crosswise to each other, on both the front and back of the core, this makes the most stable configuration because it restricts the movement of the MDF in both directions. Commercially made MDF cored sheets are made in this way.
John W.
That's interesting. I thought most folks put the stabilizing veneers' grain in the same direction, not crossways.My goal is for my work to outlast me. Expect my joinery to get simpler as time goes by.
If you were only going to put one layer on each side they would both be going in the same direction. What I was suggesting for an MDF core is to put two layers on each side with the second layer on each side at right angles to the first.
John W.
Ahhhhhh now I understand what you're saying.
Sometimes I bet you wish your posts wouldn't be read by dumb guys, only smart people who read carefully. :)My goal is for my work to outlast me. Expect my joinery to get simpler as time goes by.
Without photos or illustrations this is hard to follow, I'm just glad I got the point across.
John W.
<<"In fact, the best practice is to put two layers of veneer, laid crosswise to each other, on both the front and back of the core, this makes the most stable configuration because it restricts the movement of the MDF in both directions. Commercially made MDF cored sheets are made in this way.">>Are you talking about commmercially made plywood here? It has always appeared to me that on ply with a fiber core, there will be one layer of veneer on either side.Does this same principle hold true with HP plastic laminate? I know they sell backer sheets for this purpose, but I have always eschewed them in favor of a couple coats of fast drying varnish on the back and exposed (back) edges. I have never had any problems, but maybe because the countertops where always screwed down. Then again, maybe I was lucky.********************************************************
"It is what we learn after we think we know it all, that counts."
John Wooden 1910-
I worked in a shop that had some 4' X 8' sheets of Carpathian Elm veneered up on MDF substrate, shop owner didn't want to pay to have veneer put on both sides, stuff warped to hell and back!
I think he gave somewhere close to $500 a sheet for the stuff, boy he really saved big!
Doug
As a sample, I layed up 4 layers of 1/8 tempered masonite 3" X 24" with a nice 3" high curve using a foam mold and vacuum bag. On the outside layer I laminated 1/32 cherry veneer. I did this 4 weeks ago, took it to a trade show for a weekend where the temperature was much different than my shop. It seem not to have moved, without any warping.
When I sanded and sprayed the clear coat on, the masonote looked amazing. Almost black..The combo with the cherry was great. So maybe for a small surface area tempered masonite might be better than MDF.
Ted
3" X 24" isn't that big of a piece, wonder what would happen if you did a 4' X 8' piece that way?
IF your bagging it it isn't going to take you any longer and the cost of a flawed piece of veneer is minimal. Cheap insurance.
Doug
"...shop owner didn't want to pay to have veneer put on both sides, stuff warped to hell and back!"
He obviously didn't enough common sense use a cheap backing veneer on the other side. Say, where did he train? (Just so I can avoid more of the alumnii....)
Scrit
where did he train
I assume that's a rhetorical question!
I think we all told him that he needed it but he wouldn't budge, that is until he had 1/2 dozen sheets of veneered up mdf going to sh!t on him. But to his credit, he did learn, because on the next batch that he ordered it did have a veneer on the back side!
sometimes knowledge is expensive!
Doug
Joe
Any trade veneering shop will normally veneer both sides for stability. Adding a layer of glue and another of veneer pulls even the best of MDF around. I'd do the same on plywood rather than take chances. The reason is plain to see - in use a raw MDF face will absorb moisture from the atmosphere at a different rate to a veneered one, which is why you get warping......
BTEW MDF is probably a better, flatter, more consistent substrate than plywood. IMHO
Scrit
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