Veneer Adhesion Failure

[Ken Krawford]

I glued some 20" x 27" unbacked cherry veneer with Titebond Coldpress Veneer glue. In spite of using every clamp in my shop and 5 cauls I still ended up with a couple of areas where the veneer didn't adhere to the substrate. The one other time this happened I tried injecting some glue into the void and then clamping but the results weren't good. I don't know if ironing the areas would melt the glue or not. The veneer was applied yesterday and Titebond says to allow about 72 hrs for full cure.
Does anyone have any experience with this product regarding using heat to melt it? Any other suggestions would also be appreciated.

[Mike Henderson]

I have not had good success with the Titebond coldpress glue. As for re-gluing areas that didn't stick, use Gorilla glue (the poly stuff, not the PVA stuff). The problem with injecting PVA type glues is that PVA doesn't stick well to cured PVA.

Mike

The way to inject Gorilla glue under the veneer is to cut the veneer along the grain and then use something to carefully lift the veneer. Work the gorilla glue under the veneer with a thin rule - one of those pocket rules that you occasionally get free. Try to work the glue under the veneer on both sides of your cut. Then put a caul with packing tape on it, or put a layer of plastic under your caul, and clamp it. One problem you can run into is that the glue builds up a bit so you can have a small bump there. You can usually sand it down.

[One other comment: When applying the glue, make sure the substrate is clean. If you have any questions, run a sander over it to take the top layer off. Only apply glue to the substrate, not to the veneer. How much to apply is by experience - too much and you'll get a puddle of glue in spots and it will look like the veneer didn't adhere - too little and you'll have spots that didn't adhere. You need to apply the Goldilocks amount.]

[Kevin Jenness]

The problem is likely uneven pressure, insufficient glue spread or a contaminated substrate. If you plan on doing much veneer in the future a vacuum press is a good investment and will give consistent pressure.

As far as heating and re-pressing it, it's worth a try- what's the worst that can happen? Heat the area with an iron over a damp cloth to prevent scorching and rub it down with a wood block with an eased edge while still hot. If that fails you can try injecting polyurethane or cyanoacrylate and clamping. You can force the glue into the depths of the bubble with compressed air. Once the new glue has set, dampen the surface with a wet rag to see if the problem is solved.

[Phil Mueller]

I’m sorry to hear about the failed glue up. I have successfully used Titebond cold press for dozens of projects and have never had a failure. I’ve done some with cauls/clamps and some with a vacuum press. Never an issue.

I think Kevin is right about what may have caused it. Or as Mike suggests, too much glue and it’s actually a hump.

By the way, I rarely leave it clamped up more than a couple of hours.

[Jerry Wright]

I have used cold press glue many times with no issued. I keep matching pieces of BB plywood and trap the panel between them. I have some deep throat(6-8") clamps and clamp toward the center first, then around edges. I also have a pair of parallel clamps (woodcraft) that work on wider pieces.I place wax paper between the BB and the veneer so there is no adhesion. I find this glue to be amazingly good. I would use a hot iron on your "bubbles" and see what happens.

[roger wiegand]

I don't know about coldpress, but with regular titebond ironing it flat is very successful. It reactivates with heat and bonds very strongly. I often use it as my first choice application method-- titebond on both the veneer and substrate, let it dry, and then iron them together to bond.

[Mel Fulks]

I’m surprised that no one is using hot hide glue and veneer “hammer” Certainly much faster than clamping, except when pieces of the same
size can be stacked.

[Mike Henderson]

My problem with hot hide glue for veneer is that hide glue is water based. Some veneers expand a fair amount when they get wet. With certain veneers it's a problem to lay with any water based glue, such as PVA, and get it laid and in press before the veneer absorbs much water.

A particularly difficult pattern to lay is a radial match in a rotary cut veneer, such as rotary cut bubinga (often called kewazinga). Burls can also be a problem. As soon as the bubinga absorbs any moisture, it expands so by the time you lay all of the pieces except one, that one has to be smaller than the others and can be noticeable.

I've had to use a non-moisture based glue for certain veneers in certain patterns. For example, I did a table top in a radial pattern (could have been 16 pieces) in kewazinga. I used epoxy. It came out very nice.

But our ancestors laid burls in rotary patterns (sunburst) with hot hide glue so I assume there's a way to do it and make it come our properly.

Mike

456637

[Mel Fulks]

Mike, thanks for that explanation. I remember when white glue was quickly accepted by shop owners who had used hot hide glue for years.
They were tired of the cost of glue pots , as some did get knocked over and broken. Then they found that white glue would creep and make
perfectly flat veneer pieces move and have to be rescraped and sanded , and it took more time to set. Modern glues can block stain, so that can be a time killer when a finisher finds that and has to send the piece back to the cabinetmaker. The thinking was that getting one man to
fix a problem made by another increased the number of faults.

[Ken Krawford]

Thanks everyone (and especially Mike for the detailed response). I glued up a "test piece" yesterday after posting and left an area in the center without glue. I'm going to use that as a practice piece before proceeding.
Mike, is this the Gorilla product you recommend - https://www.gorillatough.com/product/original-gorilla-glue/? I'm afraid it is and it's one product I hate working with for a variety of reasons but will use it if it's the best solution for my problem.

[Jim Becker]

I agree with the comment above about uneven pressure. Using a vacuum press as suggested is one solution. You can also use a full size piece of material on top of the workpiece to "make a sandwich" so that clamping pressure can be better spread...that works with both "a gazillion clamps" as well as with a vacuum press. I actually keep some MDF panels for doing small laminations in my vacuum bag that are scored to permit better air extraction while going under vacuum for this purpose.

[Phil Mueller]

Jim, I have a variety of sized mdf and plywood panel cauls as well, with the corners rounded to protect the vacuum bag. I also have a few that have thin felt covering the surface. I use these when I’ve put together a veneer piece that includes inlay that sits a bit proud of the veneer. The felt helps to provide even pressure to both the proud inlay and the surrounding veneer.

[Mike Henderson]

Thanks everyone (and especially Mike for the detailed response). I glued up a "test piece" yesterday after posting and left an area in the center without glue. I'm going to use that as a practice piece before proceeding.
Mike, is this the Gorilla product you recommend - https://www.gorillatough.com/product/original-gorilla-glue/? I'm afraid it is and it's one product I hate working with for a variety of reasons but will use it if it's the best solution for my problem.

Yep, that's the stuff. I don't use it for anything except fixing veneer, probably for the same reasons you hate it. I buy the smallest bottle they make and it gets hard in the bottle before I use all of it.

Mike

[Mike Henderson]

I agree with the comment above about uneven pressure. Using a vacuum press as suggested is one solution. You can also use a full size piece of material on top of the workpiece to "make a sandwich" so that clamping pressure can be better spread...that works with both "a gazillion clamps" as well as with a vacuum press. I actually keep some MDF panels for doing small laminations in my vacuum bag that are scored to permit better air extraction while going under vacuum for this purpose.

Jim brings up an important point - you really should use a caul when pressing veneer. When using a vacuum bag, you have a bottom platen which works as the bottom caul. But you need to make a top caul that generally fits the work (just a little bit larger) and round over the top edges so it's easy on the bag. The problem is that you do work in all different sizes so you wind up with a lot of upper cauls in the shop. I keep saying that I should use some of them for scrap work but I keep them around "just in case" I have something that exact size.

You can't use a caul that's too big because the vacuum bag will pull the sides down and generally release the pressure in the center of the caul. The reason for making it a bit larger is for convenience of getting it aligned on the work in the bag. If it's "exactly" the same size as the work, you have to align the caul very accurately and even then it can move as the vacuum is pulled. So I make it maybe a !/4 inch larger on each side.

Whatever you do, make sure you put plastic between the work and the caul. Glue can come through veneer and if it does, and you don't use plastic, it will glue the caul to the veneer. Removing the caul will probably damage the work.

Mike

[Mike Henderson]

Jim, I have a variety of sized mdf and plywood panel cauls as well, with the corners rounded to protect the vacuum bag. I also have a few that have thin felt covering the surface. I use these when I’ve put together a veneer piece that includes inlay that sits a bit proud of the veneer. The felt helps to provide even pressure to both the proud inlay and the surrounding veneer.

Phil makes a good suggestion. I do the same but I have some heavy canvas that I use to spread the pressure, or sheet cork (I bought a large roll of it years ago).

Mike

[Jim Morgan]

Jim brings up an important point - you really should use a caul when pressing veneer. When using a vacuum bag, you have a bottom platen which works as the bottom caul. But you need to make a top caul that generally fits the work (just a little bit larger) and round over the top edges so it's easy on the bag. The problem is that you do work in all different sizes so you wind up with a lot of upper cauls in the shop. I keep saying that I should use some of them for scrap work but I keep them around "just in case" I have something that exact size.

You can't use a caul that's too big because the vacuum bag will pull the sides down and generally release the pressure in the center of the caul. The reason for making it a bit larger is for convenience of getting it aligned on the work in the bag. If it's "exactly" the same size as the work, you have to align the caul very accurately and even then it can move as the vacuum is pulled. So I make it maybe a !/4 inch larger on each side.

Whatever you do, make sure you put plastic between the work and the caul. Glue can come through veneer and if it does, and you don't use plastic, it will glue the caul to the veneer. Removing the caul will probably damage the work.

Mike

A caul isn't needed in a vacuum bag to distribute clamping pressure as it is when using clamps. A platen with rounded edges underneath the workpiece ensures flatness, and breather mesh on top softens any sharp edges or corners so the bag isn't damaged. What purpose does a top caul really serve?

[Kevin Jenness]

"Whatever you do, make sure you put plastic between the work and the caul."

I use 1/4" melamine coated particleboard for cauls. It's cheap and releases easily the first time. Subsequent use gets a light coat of wax.

Uneven sketch faces do need a soft layer protected by plastic sheeting for sure.

My vacuum press has a grooved bottom platen and smooth cauls sandwiching the layup. Some use a smooth platen and breather fabric over a top caul to ensure complete air evacuation.

[Mike Henderson]

A caul isn't needed in a vacuum bag to distribute clamping pressure as it is when using clamps. A platen with rounded edges underneath the workpiece ensures flatness, and breather mesh on top softens any sharp edges or corners so the bag isn't damaged. What purpose does a top caul really serve?

If you're only veneering one side, you can put that side down on the platen and not use a caul on top. If you're doing two sides, I've found that a caul on top gives a better (flatter) result than if just using the bag. One problem I've encountered without a caul is that glue can pool in one place under the veneer on the upper surface (often in the middle) - of course, this is related to too much glue but none of us are perfect. If you use a caul I've found that it avoids the pooling.

But, in general, I've found that a caul will produce a more consistent result than just using the bag. Making cauls for veneer work is time consuming and uses material so I've often done veneering without a caul. But I've also found that I get better, more consistent results with a top caul.

Another way to look at it is by pressure. With just a bag on top, the maximum pressure you'll get is about 14 pounds per square inch (for a perfect vacuum). But if you use a caul and have a high spot, the pressure spread over the caul will be concentrated in that one place. Think how a piston and connecting rod works. The pressure on the piston is taken up by the connecting rod. If the piston has an area of 5 square inches and the pressure is 100 pounds per square inch, the connecting rod sees 500 pounds of pressure. Any high spot on your veneer work (or a glue puddle) will see a much higher pressure if you use a caul than if you don't use a caul.

Mike

[Bill McNiel]

FWIW - It seems like I may be the only one here who uses Contact Cement and a J-Roller? I have used this process for 50+ years without a failure (probably why I have never sought another method).

[Mike Henderson]

FWIW - It seems like I may be the only one here who uses Contact Cement and a J-Roller? I have used this process for 50+ years without a failure (probably why I have never sought another method).

Contact cement is generally not recommended for raw (unbacked) veneer. It does work well (especially in millwork) for backed veneer.

Mike

[Bill McNiel]

Contact cement is generally not recommended for raw (unbacked) veneer. It does work well (especially in millwork) for backed veneer.

Mike

Agree with this.

[Jim Morgan]

If you're only veneering one side, you can put that side down on the platen and not use a caul on top. If you're doing two sides, I've found that a caul on top gives a better (flatter) result than if just using the bag. One problem I've encountered without a caul is that glue can pool in one place under the veneer on the upper surface (often in the middle) - of course, this is related to too much glue but none of us are perfect. If you use a caul I've found that it avoids the pooling.

But, in general, I've found that a caul will produce a more consistent result than just using the bag. Making cauls for veneer work is time consuming and uses material so I've often done veneering without a caul. But I've also found that I get better, more consistent results with a top caul.

Another way to look at it is by pressure. With just a bag on top, the maximum pressure you'll get is about 14 pounds per square inch (for a perfect vacuum). But if you use a caul and have a high spot, the pressure spread over the caul will be concentrated in that one place. Think how a piston and connecting rod works. The pressure on the piston is taken up by the connecting rod. If the piston has an area of 5 square inches and the pressure is 100 pounds per square inch, the connecting rod sees 500 pounds of pressure. Any high spot on your veneer work (or a glue puddle) will see a much higher pressure if you use a caul than if you don't use a caul.

Mike


Thank you for your thoughtful response, Mike. In my experience - no doubt much less than yours - I have not observed any consistent difference between using/not using a top caul. I roll on glue and/or spread it with a notched trowel, so perhaps I have avoided the pooling problem through sheer dumb luck. At any rate, the effectiveness of a caul in focusing pressure on high spots will be related to its stiffness - 1/4" melamine, for example, probably wouldn't do very much.

[Norman Pirollo]

Not sure if it has been mentioned, but introduce a camber into the cauls. So the part against panel has a slight convex curve to it. This worked for me before obtaining a vacuum press. Each caul is then clamped along the edges and the camber will even the pressure along its length.

Norman

[Mike Henderson]

Thank you for your thoughtful response, Mike. In my experience - no doubt much less than yours - I have not observed any consistent difference between using/not using a top caul. I roll on glue and/or spread it with a notched trowel, so perhaps I have avoided the pooling problem through sheer dumb luck. At any rate, the effectiveness of a caul in focusing pressure on high spots will be related to its stiffness - 1/4" melamine, for example, probably wouldn't do very much.

I have done a fair amount of veneering without a caul - sometimes I guess too lazy to make one. Each job seems to be a different size so you need a different caul. But I have had problems without a caul.

What I often do when I don't use a caul is roll the veneer through the bag after vacuum has been pulled. Roll it with something like a J-roller, or even a smaller roller.

Mike

[Kevin Jenness]

" breather mesh on top softens any sharp edges or corners so the bag isn't damaged. What purpose does a top caul really serve?"

I have read of instances where veneer was left with impressions from the mesh. A top caul will work to flatten slightly squirrely veneer that might not be pressed down flat otherwise. I've been using 1/4" melamine successfully for a long time- not saying it's the only or best way but it works for me. When doing any quantity of veneering a "library" of caul sizes builds up which can be resized and reused without cutting into new sheets.

I would not recommend contact for raw veneer.

[Mel Fulks]

The companies that make contact cement rate it as lasting 5 to 7 years. But ,yes, it can last longer. In commercial work
Im not sure if the workman would hear about a failure after even one year ,as the buyers would know there is no warranty
at that point.

[Phil Mueller]

Although I’ve never really experienced this issue, some say one main advantage of using only mesh is that it would allow any excess glue squeeze out some place to go (albeit small), rather than smear onto the top of the veneer (which could happen with a caul). I suppose it is a real possibility with burl or other veneers with pin holes and small cracks, however, proper glue application should minimize this.

[Mike Henderson]

Although I’ve never really experienced this issue, some say one main advantage of using only mesh is that it would allow any excess glue squeeze out some place to go (albeit small), rather than smear onto the top of the veneer (which could happen with a caul). I suppose it is a real possibility with burl or other veneers with pin holes and small cracks, however, proper glue application should minimize this.

When gluing veneer that I think I might have glue go through, I use Elmer's White Glue. You can wash it off after it cures. So if I have bleed-through, I use a wet rag and wipe vigorously and the surface glue will come off to the point were it won't show under a finish. You can't do that with most other PVA glues, such as Titebond.

And white glue holds very well, as long as the piece is not in a wet location.

Mike

[Phil Mueller]

Good to know, Mike, thank you.

[Don Stephan]

I have read that people who spray a very thin solvent based first coat after using solvent based contact cement on veneer have found the solvent in the thin first coat easily penetrates the veneer and softens the contact adhesive.

Once I was asked to fix veneer sliding down the side of a wall cabinet. The side received direct sunlight much of the day. I told the homeowner to call the company that made and installed the cabinets so I suspect but don't know if the veneer was glued with contact cement.

Regarding applying yellow glue, letting it dry, and then using an iron to glue veneer to a surface - after reading about it in FWW I decided to take that approach on a tapered sculpture stand. The olive ash veneer was buckled so first I used flattening solution and a vacuum bag and newspapers to dry it. Likely it was not completely dry. The next morning after ironing on the veneer there were several small gaps between the bookmatched pieces of veneer on all four sides and the top. Not having an eye for color, I spent three days with shellac and powdered pigments making the filler disappear.

[Bill McNiel]

The companies that make contact cement rate it as lasting 5 to 7 years. But ,yes, it can last longer. In commercial work
Im not sure if the workman would hear about a failure after even one year ,as the buyers would know there is no warranty
at that point.

Mel,
I had never heard this before and it obviously shocked me so I contacted multiple former clients to check up on the current well being of my work. All eight of the people contacted were surprised by my inquires. The average reply was, "Hey, X years (10 to 40) later you are questioning your process? The piece is as beautiful as the day you delivered/installed it other than normal wear and tear."

Back in the day there was no internet to check reviews or gain backdoor info on products so we just did what we learned/knew and hoped for the best. I can't remember any manufacturer advertising their product to have a limited effective life span, but that does not necessarily make it reality. Ignorance is bliss or I have been very fortunate.

Thank you for alerting me to this issue, I will be contacting Weldwood in the near future.

Regards - Bill

[Mel Fulks]

Bill, that "5 to 7" year quote was from on line info ,I think from one of the manufacturers. I don't remember which.
In employments I've heard many discussions about call back lists. And who was who would be involved in the patching.
Loose at corners is always a big one. Round off corners seem to fare much better. The Weldwood plastic resin glue works
well. It brushes on better if you let the mix sit about 10 minutes before using, without that it takes more brushing to
coat spots that resist being coated.

[Joe Hendershott]

Thank you for your thoughtful response, Mike. In my experience - no doubt much less than yours - I have not observed any consistent difference between using/not using a top caul. I roll on glue and/or spread it with a notched trowel, so perhaps I have avoided the pooling problem through sheer dumb luck. At any rate, the effectiveness of a caul in focusing pressure on high spots will be related to its stiffness - 1/4" melamine, for example, probably wouldn't do very much.

We use a vacuum bag with NDF on the bottom and breather mesh on top nearly every day. Never an issue. Use a 1/32 inch trowel to get nice even coverage with Titebond cold press or Better Bond and it's a simple easy process.

[Bill McNiel]

Mel,
Thanks for the info, I truly appreciate the input. SMC provides a great deal of real life experiences re. tools, techniques, materials, etc. generally without a noticeable bias that have proven to be very helpful to an old fart who doesn't want to be "stuck in his ways".