Adhesives and Methods for Very Thin Glue-Ups

This discussion of how to glue up wide boards and re-saw them to 1/8 thickness includes some technical info and insight into glue types and the nature of glue bonds in wood joints. February 4, 2011

I need some wide wood, 1/8" thin. Maple and red oak. I'm looking to use 2 or 3 boards to achieve 10" wide. If I glue up some 4/4 stock, then resaw and plane and sand to 1/8", will the glue-ups hold? Assume Titebond glue. Indoor usage.

Forum Responses
(Sawing and Drying Forum)
From contributor T:
The glue joint, if properly done, will be stronger than the wood itself. Properly done means true 90 degrees on straight edges with adequate glue coverage, proper drying conditions, etc. Do not know what usage will be, so cannot comment on expected success of your project.

From the original questioner:
Typical usage will be scroll sawing, sawing puzzle parts, and other toys.

From contributor L:
10" wide boards are available. May cost a bit more. Glued up should also work fine with a properly done glue joint, as mentioned above.

From the original questioner:
Yes, I can find 10 wide boards, but narrower stock is much more prevalent. Plus glue-ups reduce risk of warp.

From contributor G:
First, gluing up one inch lumber to make 1/8 inch panels is overkill and more likely to result in glue voids that will fail than starting with thin wood, and a waste of at least two thirds of your wood. As far as thin wood glue-ups, yes, they will hold. The wood will fail ahead of the joint. I build musical instruments and routinely do soundboard glue-ups of 36 inch width.

A book on guitar or harpsichord building will show you the tricks. Mostly you need straight, square edges (or edges planed in pairs so that any unsquare is compensated for). Clamp lightly so they don't buckle, or weight or cross tape the joint. Usually I just cover a board with waxed paper and put a pair of cleats down. Arrange one side slightly off square so that you can use a long wedge to apply pressure. Very little pressure needed, or you will squeeze out all the glue. For your purposes, Titebond will do. For instruments we use traditional hot hide glue (less dampening of sound - hide glue dries hard). Added advantage with hide glue is no clamps. You rub the glued joint together for a second and hold it for a few more and after the glue has gelled, you can just leave it alone to dry without clamps.

By the by, this is all done with wood 3 to 5 inches wide and a touch over 1/8th inch. Resaw your 1 inch boards to about 3/16ths and then surface to a touch over 1/8th using either the table saw or a bandsaw. Join them up and then run the panels through a sander. (For soundboards no sanding, you use a smoothing plane for final adjusting.) Wait until the joints dry fully or you will sand away the swollen wood along the joints and when it does dry you will have dips along the glue line.

The only problem you have with thin wood is splitting if you restrain it. On a harpsichord soundboard three feet wide and five long, one must glue the perimeter to ridged supports all around. If you do not either wait for a very dry day or pre-dry the soundboard before gluing it in, then when the humidity drops, it will split along the grain (but not on the glue lines). A three foot wide panel can have 1/4 to 3/8th inch movement over humidity extremes. So you either need to control the atmosphere the instrument will live in or decide if splitting or buckling is easier to live with. If your usages are small items or you do not restrain the width, this will not be a problem. (I have a friend who is beginning to make puzzle boxes and is having great trouble with parts sticking as the humidity changes.)

This video shows a guitar top glue up that might help with the gluing and clamping concepts a bit.

From Professor Gene Wengert, forum technical advisor:
In any case, it is super critical that the surfaces be perfectly flat, and less than ten minutes old. We can have voids in a large surface, but not in this small one. I have seen where a hot glue is used as a temporary clamp.

From the original questioner:
Gene, are you saying I will get better holding power if I glue shortly after making my cuts, versus getting my edges smooth and true and then gluing the next day?

From Professor Gene Wengert, forum technical advisor:
Get them smooth and flat, then glue within 10 minutes. A small edge like this will deteriorate quickly, so you need to move fast.

From contributor G:
Gene, I do not disagree with you, but I have not noticed a difference between fresh cuts and not so fresh ones. I usually glue right after jointing, but I am interested in this deterioration you mention. Drying? Grease in the air? And why does it affect thin wood in a disproportionate manner? Is this a personal experience or would you be able to point me to a research study that supports this?

Quite frankly it seems counterintuitive. A square inch of gluing surface is a square inch of gluing surface, and a joint one inch square contains the same surface area as a joint 1/8th inch thick by 8 inches long. Those forces present in lumber one inch thick that tend to resist gluing (twist, warp, bow and so on) are eight times greater than those in a 1/8th inch joint... while offering eight times the surface area involved in the joint to resist those forces. It would seem a wash either way.

The only effect I notice is the smaller volume of glue chills faster on a thin board than a thick one, so you need to get the joint together a bit quicker before the glue gels... but that is not related to the age of the surfaces. I shall look forward to hearing your argument.

From Professor Gene Wengert, forum technical advisor:
The aging of surfaces is a well know event. It is called surface inactivation. It results because the chemical magnets (like OH-) on the fresh surface are trying to find something to satisfy their need. So they will attach to water vapor, dust, dirt, etc. that is in the air (and maybe even oxygen).

With a large surface, we can tolerate some loss of strength in the joint as the good areas will still be strong. However, with a thin joint, weak areas will have a big effect. Also, many times, the strength that a joint must have is perhaps 75% of the wood's strength, so that the joint can lose half its strength and still perform well. With a thin piece, we probably need 100% of the strength, however.

In addition to texts that discuss this, I have had many situations where I have seen its effects firsthand. Note that in addition to surface inactivation, a moisture change can also cause the surface to change size and then there can be a gap over 0.006" which leads to poor strength.

Just one example: I worked with an RV plant in IN that made its own cabinets. They had about 10-15% failure of the edge-glued panels in the wintertime, which meant they had to repair. I asked them, before I came for a consulting visit, to make 50 panels, but apply the glue within 10 minutes after machining the edge. They did and had only one failure. In addition to canceling my visit (which means I did not get paid as a consultant), they did let me know that they no longer have any issues since they went to rapid gluing after machining.

From contributor G:
I shall keep it in mind. As I said I usually do the glue-up immediately after anyway. I will note that in several dozen (a number approaching 50) books on instrument construction, including harpsichord, guitar, violin, pipe organ, etc., I have not read one word suggesting that there is any need to glue on such a schedule. Do you imagine the chemical deactivation is less of an issue with traditional hot hide glue?

From Professor Gene Wengert, forum technical advisor:
It is possible to get good strength in a glue joint due to the geometry of the surface... the nooks and crannies. It is kind of like jigsaw puzzle pieces. If high strength is not needed, then a partially inactive surface is okay. I do believe that hide glue can span a small gap, so that would also eliminate the size change issue mentioned.

From the original questioner:
Well, most of this 1/8" stock will be going through scroll-sawing... so I'd call that pretty stressful. Are you saying hide glue would be best, and is Titebond hide glue?

From Professor Gene Wengert, forum technical advisor:

From the original questioner:
I thought TB2 was the same as TB, except it was water resistant? And since I won't be using these boards outdoors, or exposing to water, I'm not sure why TB2 is suggested?

Form contributor G:
Titebond is a plastic, I believe polyvinylchloride based material (I, II, III and all variants). Hide glue is entirely organic, made from the skin, bone and assorted connective tissues of various animals, usually cow. (Same stuff as gelatin.) It is supplied in granules or sheets and is prepared by adding water, allowing it to soak and the next day heating it to 140F. It is the traditional glue used as far back as ancient Egypt and almost exclusively up to the beginning of the twentieth century. It is vastly preferable to PVC for musical instruments as it dries hard, not rubbery (and therefore does not damp sound nearly as much) and it does not have an appreciable amount of cold creep as PVC does. On the down side you need to prepare it fresh, keep it hot, use it on warmish wood and get the joint done before it gels. It can be reheated within limits to affect a joint and can be easily reopened if needed, even years later, with a bit of moist heat. Many joints can simply be held in place until is sets (without clamping) and it is the basis of the simple process of hammer veneering. There are some tricks to extending its open time, but the one used to produce the commercial liquid hide glues (which never gel) vastly reduces the good properties of hide glue. However, for general scroll saw work, I would just use one of the Titebond varieties.

From Professor Gene Wengert, forum technical advisor:
TB II has a high instant tack (stickiness). It does dry fairly hard, as it will quickly dull sanding belts. The early PVAs did stay soft, but TB II is an aliphatic PVA so a chemical reaction occurs, making it hard to soften with heat or moisture; the early PVAs were easy to soften, but not the newer ones. Creep is not an issue unless the joint is under high stress.

From Professor Gene Wengert, forum technical advisor:
Incidentally, wood is a plastic material, yet it is used for musical instruments. Nothing wrong with plastics.

From contributor G:
My point was that PVA glues, in soundboards and bodies of acoustical instruments, represent a substantial loss of acoustical energy when compared to hide glue due to the more rubbery nature of PVA.

Most of luthiery also excludes plywood because of its acoustical losses from the fragmenting of the transmission path of the energy through layers of wood and glue. (Yes, different glue, but similar problem.) On the other hand, the folks who build speaker cabinets adore ply and similar chip, particle and flake boards because they want the mechanical structure of a speaker enclosure to be non-resonant and acoustically dead.

I join you in recommending any of the PVA (Titebond) products as adequate for 1/8th inch fretsaw stock, and I reserve my right to my prejudice for hide glue, which is not based on any belief in the superiority of it as an organic or green material, but because it dries flint hard and doesn't creep.

From Professor Gene Wengert, forum technical advisor:
We do need to appreciate that TB II is indeed a PVA, but it is not like the old PVA adhesive we called Elmers glue, which did creep under load and did soften with heat and moisture. TB II is quite a bit different. It does not creep. It does not soften with reasonable heat. It withstands moisture. Tests for creep have been negative at reasonable load levels.

One reason to use a hide glue, which is not the stuff sold as liquid hide glue in a bottle, is that the glue is brittle, so if an instrument is dropped, the glue joint will break and not the wood. A second reason, especially for repairs, is that with heat, the glue softens and the joint can be more easily repaired. With TB II, the joint will be hard to ever repair, so the instrument, when damaged, may have to be tossed. That is, when damaged, the instrument cannot be taken apart, the defective piece replaced and then put back together. We do see many guitars made today using PVA?

From contributor B:
One day a pair of woodworkers were getting a dovetailed box together. They had the mallets out and were really forcing the issue. When they finally had it together, one panel had cracked from the stress. When I asked about their troubles (from a distance - they were frustrated and still had a hammer), they admitted they had milled the dovetails two weeks earlier. The wood had moved in that time, making the fit impossible without their efforts. In the end, they as well as I learned to do the glue-up work as soon as possible.

Thanks, Gene, for the info on surface inactivation, and to all others for their additions to this thread; very informative.

From the original questioner:
I see the TB website has a very nice checklist of desired properties to assist in glue selection. I'm planning on resawing these glue-ups after the bond has cured, down to 1/4" and 1/8" thin. Since I will have cleanup after resawing, I was concerned about Gene's comment that TB2 will dull sanding belts.

So, I selected the following properties from their property list:
Slow set - 5 minutes
A moderate assembly time
Excellent creep resistance
Excellent sandability
Dries clear

Their suggestion was liquid hide glue. They said it has great sandability, moderate assembly time, and excellent creep resistance. Second on their list was a product called HiPURformer MP300, which I'm not familiar with. Third was their polyurethane glue. I did note they state their TiteBond II Dark has excellent sanding properties, so it looks like its color is not the only difference from the non-dark TB2.