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Necessary to veneer both sides of a 1-1/2” thick laminated plywood substrate?

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Forum topic by IsaacJames posted 10-12-2018 03:15 PM 3131 views 0 times favorited 28 replies Add to Favorites Watch
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IsaacJames

6 posts in 61 days


10-12-2018 03:15 PM

Topic tags/keywords: veneer tabletop laminating plywood question

Hello all. I am a professional woodworker building a custom table for a client and I need some opinions on a question I just cannot seem to answer for myself.
This client provided me with a doable yet limited supply of Walnut needed to build her a trestle table. She wants a rather elaborate tabletop layout so I convinced her to allow me to make 1/8”- veneer out of some of the material for the interior portions of the tabletop. The edges as well as the breadboard ends will be solid Walnut but the interior components will be the veneer glued onto (2) 3/4” sheets of plywood that will be face-glued together, giving the tabletop a solid feel and a girthy appearance. (As per her request) I am fully aware of the “always apply veneer to both sides” rule, especially when dealing with thin plywood/MDF. However, considering that I have a limited amount of the provided Walnut and considering that the substrate will be roughly 1-1/2” thick, do I need to spend the time and the energy veneering the underside of the substrate in this instance? (The veneer is rough sawn so I imagine the thickness will be down to 3/32” or thinner after planing, scraping and sanding.) Your thoughts, opinions and experiences are welcomed and appreciated.
Thanks!

-- | Measure Once, Cut Twi |


28 replies so far

View TDHofstetter's profile

TDHofstetter

4 posts in 61 days


#1 posted 10-12-2018 03:32 PM

If you don’t add some sort of veneer to the underside, then the plies won’t be balanced and any humidity shift may encourage some warping. Even if you need to buy some nasty junk veneer for the underside, veneer it. The surface plies should always be the same thickness and in “grain agreement”.

-- -- Tim --

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DS

3030 posts in 2620 days


#2 posted 10-12-2018 04:14 PM

When laminating you should ALWAYS make balanced panels.
It is important to have the same treatment on the front face as the back face.

No matter how strong you think your substrate is, an unbalanced panel is doomed to failure.

There are plenty of acceptable backer sheets that can be used instead of expensive veneers in order to balance your panel.
Polybak sheets are super stable and relatively cheap. (About $9 per 4×8 sheet in my area)

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

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Andybb

1490 posts in 803 days


#3 posted 10-12-2018 04:56 PM

As you said, you are a pro and I am definitely not but …. 1 1/2” seems pretty darn thick to worry about veneering the underside. Especially since the underside is a completely different sheet of ply from the top side. Almost as if the plywood is 3/4” veneer. Good chance that I am wrong but I would think that veneer attached to a seperate sheet of plywood would have very little connection to the veneer on the top.

Interested to hear responses.

-- Andy - Seattle USA

View oldnovice's profile

oldnovice

7326 posts in 3567 days


#4 posted 10-12-2018 06:10 PM

I was taught and always followed “balance” whether veneer, finish, or paint.
Hasn’t let me down so far!
But that was 60+ years ago …. maybe things have changed?

-- "I never met a board I didn't like!"

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Aj2

1872 posts in 1997 days


#5 posted 10-12-2018 06:30 PM

Of course you should treat both sides equally.What could be the downside the cost of a cheap veneer and the time it takes. Hardly worth the risk of a cupped panel.
I’m sure there’s ways around using ridged glues but then there a whole new set of issues that come up.

-- Aj

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DS

3030 posts in 2620 days


#6 posted 10-12-2018 06:36 PM

It is a known fact that wood is a natural material that moves and breaths with changes in temperature and humidity.

It wouldn’t matter if your substrate is 12” thick, if there are uneven stresses in the panel, it will move accordingly over time.

I am not saying it wouldn’t take a bit longer, but it WILL move – 100% guaranteed.
Will it stay straight and flat as it moves? That depends if it is balanced or not.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

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gwilki

226 posts in 1673 days


#7 posted 10-12-2018 07:10 PM

I don’t have an answer, but I hope you don’t mind a question for all of those who say that you need to veneer the bottom.

Since the top is made up of two sheets of plywood, what is the difference between a piece of veneer that the OP would add and the outer “veneer” or ply on the sheet of plywood in terms of how moisture will affect the piece?

I’m not arguing one way or another, and I am not a professional by any stretch of the imagination, but I really would like to understand this. The outer ply on the plywood is really just a sheet of veneer, is it not? It may be a bit thicker than some veneer, but other than that, it’s just a thin layer of wood, just as veneer is. So, why would adding another piece of veneer to the plywood substantially change the way it is affected by humidity?

-- Grant Wilkinson, Ottawa ON

View MrRon's profile

MrRon

5192 posts in 3443 days


#8 posted 10-12-2018 07:33 PM

Kitchen countertops have a laminate on one side only, yet they don’t warp.

View Aj2's profile

Aj2

1872 posts in 1997 days


#9 posted 10-12-2018 07:37 PM

When the plywood was made they veneered both sides correct.? The smart guys know without doing this the panel would warp straight out of the press.
Now the op wants to upset this balance so it needs to be equally treated on both sides.

I will now attempt a scientific guess to what happens when we add water based glue to One face of a sheet of plywood

Glue is absorbed into sheet, the cells puff up into their natural shape everything is happy. Panel is removed from clamps air is now in contact with surface. And panel starts shrinking as water starts leaving unhappy cells. A bazillion cells all shrinking a tiny bit adds up and pulls the whole substrate into a unknown shape that’s not anywhere near flat.
I rest my case :))

-- Aj

View Fred Hargis's profile

Fred Hargis

5174 posts in 2693 days


#10 posted 10-12-2018 08:38 PM

I suggest you veneer the bottom, but nothing says it has to be walnut. After all, if you didn’t veneer it , wouldn’t it be what ever the plywood veneer is? There are “backer” veneers made just for this, get some of that and use it.

-- Our village hasn't lost it's idiot, he was elected to congress.

View MrRon's profile

MrRon

5192 posts in 3443 days


#11 posted 10-12-2018 09:26 PM



Kitchen countertops have a laminate on one side only, yet they don t warp.

- MrRon


On second thought, counter tops are not plywood, but MDF or particle board.

View clin's profile

clin

954 posts in 1195 days


#12 posted 10-12-2018 09:53 PM

I agree with adding a similar veneer to the bottom. Though I don’t think it needs to be expensive wood or even walnut. Though I would use a wood with similar expansion characteristics to walnut and the same thickness as the top. And of course this would need to be cut similar to the top veneer.

Also, when I think veneer, I think of something thinner than 3/32”. I think 3/32” is thick enough to matter.

Concerning the two layers of plywood, run their outer grain the same direction. I think some might think that you should alternate them, just as the inner layers are, but it won’t be balanced that way. Running them the same just means the inner layer will be that much thicker and effectively have the desired odd number of plies.

I also can see where this might be a belt and suspenders type thing. But you are not manufacturing a thousand of them, you are making one. And you have to get it right the first time. So best to err on overdoing it.

-- Clin

View IsaacJames's profile

IsaacJames

6 posts in 61 days


#13 posted 10-12-2018 11:07 PM

Thank you all so much for your responses, I really appreciate it.
I agree with everything you all said. I completely understand wood movement in reference to veneer and plywood. I agree that veneer should indeed be applied to all sides. That being said, I also see how the second sheet of 3/4 ply could indeed act as a veneer itself considering it would be glued the same way as the top layer of walnut veneer.
However….begrudgingly I have decided to add a layer of cheap veneer to the underside of the bottom piece of substrate as an insurance policy.
Now, let me ask you all another question:
My client would prefer to have to table slightly distresssed looking. She doesn’t want it to look like a brand new piece bought from a factory showroom.
So, instead of applying the 7-1/4” wide X 32” long strips of Walnut veneer to a sold sheet of plywood, what about ripping the top layer of plywood into matching 7-1/4” X 32” pieces, glueing the veneer to them (top & bottom) and either glueing or cabinet screwing those individual veneered strips of plywood to the bottom piece of solid 3/4 ply? My thought in doing this is to create a “pieced together” effect to the tabletop as if it were built out of individual boards vs. glueing all of the veneer pieces to a single large piece of plywood giving it that typical smooth veneered look.
Do you think this would give me the effect I’m looking for or do you think this would just create a cluster of other problems?
Aaaaannd….......GO!!

-- | Measure Once, Cut Twi |

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IsaacJames

6 posts in 61 days


#14 posted 10-12-2018 11:48 PM

@ DS:
I really like your quote!

-- | Measure Once, Cut Twi |

View shipwright's profile

shipwright

8165 posts in 2997 days


#15 posted 10-13-2018 02:30 AM

First: I would, to be safe, put some kind of veneer on the back.
However, if it were me, I’d be doing a sample to keep around the shop with veneer only on one side.
I’ve done a lot of veneering and I don’t think this will warp. You have at least 14 glue lines. The more layers in a lamination, the more stable it is.
Gut feeling, I don’t think it moves at all.

-- Paul M ..............the early bird may get the worm but it’s the second mouse that gets the cheese! http://thecanadianschooloffrenchmarquetry.com/

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