Gluing Up Thick Turning Stock
Cracks may open up at glue bonds in large glued-up pieces, particularly in outdoor exposures. Here are some ideas for high-performance adhesives that may help. October 20, 2005
I sometimes have problems when gluing up 24/4 or thicker stock for turning squares. While all the joints may appear to be tight when the work is clamped, some of the glue lines open up later. It appears that the moisture from the glue is expanding the glue side of the outer layers, creating stress in the form of cupping that opens the joints. I learned the hard way that Titebond II is not structurally rated when I had problems with square turned porch posts and staved porch columns with glue lines opening up days or even months after turning and installation.
I've used structural epoxy to get around this problem but this is rather expensive. I have heard for years that you can starve a glue joint by using an insufficient amount of glue but I wonder if putting too much glue on would cause the theoretical wetting/tension problem. I am looking for advice on a cost effective adhesive for gluing up thick turning stock. I have also noticed that the harder woods prove more challenging to glue up into stable stock.
From contributor A:
I have noticed over the years that fat solid turnings do not age well in interior environments, and ditto/double for exterior work. The unavoidable humidity cycling will eventually cause cracks to open up in the wood, if not glue lines. The mechanism is shrinkage, similar to the way a whole log cracks as it dries. A 24/4 chunk works out to about 6", and I would question the MC starting out. If it is high for your area, then no glue will hold it. I try to hold solid exterior to 4" or less and hollow above that. Interior can be up to 6" solid, and hollow over 6". I use epoxy or resorcinol glue for exterior. I'd rather stay on the safe side.
From contributor B:
Have you tried polyurethane construction adhesive that comes in a caulking tube, such as PL Premium? I have been using it for years with excellent long term results. It doesn't grab like regular wood glue and takes longer to cure, about 3 hours minimum. It is a rigid plastic when cured and sands well. When sanded it also matches wood and stains well. It is also 100% waterproof.
From the original questioner:
I have used PL but mostly for repairing old columns with the idea that its gap filling qualities are beneficial and it is easy to trim the squeeze-out flush after it has cured. I have mixed success because of the varying conditions of the wood surface. I have never tried it on new clean wood but I may on some future paint grade project.
From Gene Wengert, forum technical advisor, Sawing and Drying Forum:
A hot-melt PUR would be ideal. You get good gap fill and high strength. The PUR (PolyUrethane Reactive) adhesive is a 100 percent solid, one-component urethane prepolymer that behaves like a standard hot melt until it reacts with moisture to crosslink, forming a new polyurethane polymer. By curing the polymer in this way, PURs have performance characteristics that are more enhanced than those of standard hot melts.
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