Not to beat a dead horse, I saw this on Wood Boat Forum and thought it was somewhat relevent to this discussion so I am posting this here...
Over the years I have several times worked as a technical advisor in the wood processing industry, that includes fingerjointing, laminating, plywood manufacture and both particle and mdf boards. All of which use glues, and all of which gave me access to very good testing equipment. I've used pva [titebond] a lot, standard white, aliphatic and cross linked catalysed. I found no difference in strength, PVA tests consistently higher in strength than any other wood glue . In my testing I made a series of control pieces using WEST epoxy, and called the mean of my test run 100%. On that base using the same softwood test material,same surface prep, same temperatures and so on, PVA came in at 120%, Phenol resorcinol formaldehyde at 112% MUF and UF at 105% Epoxys varied slightly but generally the 1/1 and 2/1 premixes were less strong than the 4/1 or 5/1 , and ALL of the polurethanes [Gorilla/Pro Bond] at between 50 and 65%.
Repairability was not brilliant in any case except for the epoxies where good surface prep brought the new joint close to the original in strength, but all of the other glues lost significant strength in glueing a previously coated joint unless planed or scraped off to a new surface. PVA is not unique in this.
Like most glues that we use, PVA is a mechanical bonding agent, it penetrates the cellular structure of the wood surface, sits in there and hardens off. The result is like the monkeys fist in the bottle in that there are lumps in the cells too big to pull out, and its connected with the monkeys fists on the other face of the joint by the glue film. Dead right it doesnt stick to itself once cured, even more so with the cross linked version so repairing a joint by smearing it with more glue is a waste of time.
So whats the issue? Good working practice is to prepare the joint with a freshly planed or cabinet scraped joint, not sanded as that can in some instances be not as good a planed surface.
All the best for the new year.
John Welsford, enjoying the summer warmth in New Zealand