Dan

Quality of fit is another story. Clamping pressure is only needed to overcome gaps between bonding surfaces… Which raises another important point: What is “gappy” to a PVA wood glue? Ideal adhesives spread thickness is between 0.006″ to 0.009″ (6 to 9 mils) and these correlate exactly with the gap filling ability of a PVA. Bob is clearly off on at least one point, the polymers do not bond directly with cellulose. From a strictly technical standpoint, they experience weak electrostatic interactions with the cellulosic interface of wood… Once again, helping to explain why fit is so essential to a strong joint. Beyond 10 mils or so, the PVA starts to behave independently of the joint (ever seen a ridge of glue that started off smooth?)

On another note, good luck finding much polyvinyl alcohol in a wood glue. While it would work fine, products like Titebond are based on polyvinyl acetate instead.

Instead of worrying about burnishing, I’ve found most people want to know more about exotic and oily woods. These follow the same rules with respect to fit and finish, but they are sensitive to other factors. PVA and hide glues are both water based, which is exactly why they have trouble bonding with an oily surface (literally mixing oil and water.) There are two good solutions to this: glue up within two hours or so of planing/sanding the wood or wipe it with acetone prior to bonding.

I have never seen a study on the use of liquid hide glue for hammer on veneer applications. Did Bob elaborate on how this was studied? Liquid hide glues work by using gel point depressant additives, which allow them to remain stable at room temperature. When heated to 110°F to 120°F they start to behave like hot hide glue, and might work under such conditions… But I have never seen or heard of it being tested.

To address some of the questions above:

Always glue to bare wood whenever possible. Sanding sealer will render the surface less porous and can be expected to interfere with the drying process and bond formation.

The reason the question of “roughing up” a joint comes up is that people are under the misconception that doing so will increase the bonding surface area and therefore improve adhesive performance. This is actually true for some 100% solid products such as epoxies.

Polyurethanes like Gorilla glue have similar needs to water based wood glues when wood is the substrate in question. In fact, their performance can be damaged even further by the presence of bubbles in the glueline.

To Steve: You might want to direct your questions in need of an expert elsewhere in the future. I understand Bob has been in the industry for a little over a year and hasn’t worked with wood since junior high shop class. His response seems to support what I’ve heard. If you must go through a company such as Titebond, that may be biased, I suggest you contact a different senior technical specialist… Mark Roberts, who has been with that group for 15 years or so. If you want to avoid industry conflicts of interest, feel free to contact me. I’ve been considering writing a book on the topic of adhesives as they relate to woodworking and have over 20 years of experience in the field. A series of articles could prove to be fun.

Just to add a word from the world of wooden aircraft, the FAA will not approve a sanded surface for gluing. All surfaces must be freshly cut or planed prior to gluing.

As an old A&P (airframe and power plant) inspector once asked me,
“Do you get better traction on a gravel road, or pavement?”

Tom