Hi,

We are making tabletops that have a veneered panel at the center and a mitered solid wood frame all around it.

The center panel will be 15 mm (3/5") Baltic birch plywood with 2 mm (5/64") shop sawn veneer on both sides, pressed in a cold vacuum press.

Veneer this thick is something in between solid wood and "true" veneer - it will exert some forces due to humidity fluctuations but those will be limited.

I did some research (reading literature, online forums, contacting glue distributors) and got conflicting advice on what glues to use.

Some recommend a rigid glue line (e.g. UF, epoxy) which is supposed to inhibit a tendency to move separately from the plywood core (which only moves approx. 0.015% per every 1% change of moisture content).

Others say that a rigid glue line could fail due to different expansion/contraction rates of the two materials and therefore recommend a more flexible glue line (e.g. PVA).

We prefer to use a glue that results in a more rigid glue line - UF or at least some kind of PVA glue that stays relatively hard at room temperature - to prevent seams form opening up in winter months.

Is there a consensus on what glues are best to use when gluing very thick veneers to very stable cores?

Also, if a rigid glue line is the way to go, would a glue like Titebond Original be suitable? When completely dry it is surprisingly rigid (brittle when bent) compared to all other PVA glues that we tried. Not sure how it compares to UF or epoxy resins though.

Paul

i do this very thing fairly frequently, and would strongly suggest a urea resin glue. i use Unibond 800 with great success.

There is no consensus on this forum, however, there is plenty of successful techniques gluing up veneer under 3mm with a variety of adhesives.
You are balancing the layup which is key to success.
Vacuum bagging presents an added difficulty in that the veneer spends a couple of minutes wet before being pressed. During this time the veneer has a chance to expand before being pressed and locked into position on the substrate. The bottom typically is rolled out first and has longer time to expand than the face. This may contribute to cupping of the top with glues having a higher moisture content. As the moisture dissipates the veneer wil pull more on the bottom. It would present as convex top surface.
Unibond, epoxy or UF have little to no water so there is no potential for moisture differential (from the adhesive that is).
I am not a believer of the rigid glue line theory. Creep, I feel is a misused term often in woodworking especially in veneer work. I have never seen evidence of it in my or others' work. Good adhesion requires a measure of flexibility. A successful glue line in pva is less than .001" thick. It wil not behave like "dry" puddle of squeeze out which is easily stretched and flimsy.
One of the most important parts ensuring flat pressings for me is how I treat the parts coming out of the press. I am careful to provide equal air flow to both sides and in winter protect the parts from direct heating air.
Finally, I have been pressing with a type 2 pva for more than ten years with predictable results. I chose the water resistant glue so I could remove my veneer tape with a wet sponge and not having the veneer bubble. The glue is easy to clean up and I don't get a constricted throat and runny eyes from the UF of gassing from powdered uf glue I used in a previous shop

Consider which of your glue options have water and which do not. Water based glues will move into veneer and cause them to expand a bit, then dry out and shrink back - again just a bit. Solvent based glues do not do this.

When we evaluated the glues at hand, we moved to epoxy. We have had no problems at all with movement.

In my opinion, epoxy has the best combination of properties for this application in the low pressure of a vacuum bag. My experience with PVA glues on thick veneer was not good, joints opening/curling.

John

Great information everyone, I have never used an epoxy adhesive for this type of application, I was wondering what brand of epoxy you are using or recommend?
Thank you.

Does epoxy peel off vacuum bags easily on cured? What precautions do you take when you veneer with it with respect to squeeze out and glue bleeding through veneer?

Epoxy

On yachts we use typical West System epoxy to bond down 1/4” teak decks to composite decks(carbon or glass fiber).

The teak is 100% restrained on the bond side. It’s exposed to soaking wet, sun cooked dry. All in the same day.

The boatbuilders had by 1990 perfected using epoxy in every conceivable way with all materials except the common plastics. The plastic bonds have been good for the last 10 years.

Likewise typical vacuum bagging has not changed since about 1990. There were a few improvements in the 90’s and 2000’s that relate only to composites.

The vast majority of woodworkers are using methods invented 30-40 years ago. I’m a architectural woodworker & boatbuilder(composite & wood).

Under the for what it's worth category...

I would suggest a UF resin such as Unibond. It's easier to use than an epoxy and less expensive. It allows for water clean up and provides a rigid thermoset bond. Unless you're building a boat, epoxy is overkill in my opinion.

The problem with Unibond 800 is that it is water based. I've had good luck using it and Plastic Resin Glue, even with 1/4" thick veneers on a stave core substrate, but Baltic birch has such low seasonal movement that it adds risk that epoxy inherently eliminates. Epoxy is no harder to use than Unibond 800. Cleanup is safe and simple with white vinegar. My two cents.

John

I am on the side of Epoxy. West System or Total Boat.
Overkill or expense be damned.
So, you are standing there in your shop, with that $21.76 you saved on glue burning a hole in your pocket. But instead of going to the wood store to spend it, you have to figure out how to fix those bubbles in the table top you just got the phone call about. Enjoy those savings.

I have had problems with about every major glue except for epoxy. Only one bubble with Epoxy. Multiple problems with Unibond, Plastic Resin, Urethane, etc.

Thanks a lot for your input. I believe we will give Epoxy a shot for this particular application.

A few more questions if you don't mind.

Should we sand our custom veneer for bonding with epoxy (if so, what grit) or will epoxy bond properly to a planed surface?

How does one tell when there's too little or too much epoxy applied to the substrate? When using PVA we judge this by the opacity of the layer.

Another question - how does one prepare surfaces for glue ups when those surfaces were machined weeks or even months ago? We used to do custom work only so we glued everything hours or days (at the latest) after individual parts were machined. Now we are launching our own furniture line and work in batches, meaning that some parts have to wait for other parts to be ready or we may have left overs from previous batches that could be used. We have a full climate control and acclimatise materials properly so that parts do not change in size/shape over time while inside our workshop. We are, however, concerned about glue bonds when surfaces are not freshly machined.

Our materials are maple, ash, oak and Baltic birch.

Thanks,
Paul

Test glue several joints - some with fresh surfaces, and some with months old surfaces. Record it all, then stress the joints to see how/when they fail.

David, that is a very good idea, we will try several different intervals - just machined, machined 3 days ago, 1 week ago, 1 month ago and 3 months ago or so.

Does anyone have any idea on surface preparation when veneering/laminating with epoxy - planed surface vs sanded surface? And the right amount of epoxy to apply for this application?

Appreciate your help!

Paul

We do a quick pass with 80 grit on a 6” Festool to provide more “tooth” to the
substrate.

I use type 2 PVA because its quick, easy and reliable - no worries about mixing, temperature, starved joints etc. I use a custom notched spreader to insure a minimum consistent glue line, All flat veneers get pressed for 30min.

While a balanced end product is essential, a case for doing the two sides 12hrs apart could be made (re: Tom's post and something I do on large layups). The example images show bowed substrate and cover sheet coming out of the press, both flat 12 hrs later.

View higher quality, full size image (1280 X 960)

View higher quality, full size image (1280 X 960)

View higher quality, full size image (1280 X 960)

Thank you all for your responses so far!

After you tape your veneers, do you glue them together (by filling seams with glue) before gluing the whole assembly to the substrate?

Are there any real advantages of doing so? Would this protect the panel from accidental water ingress? Would this prevent small gaps from appearing in extremely dry environments by holding veneers together throughout their thickness (not only at the substrate level)?

Paul