Gluing Wet Cedar for Exterior
From Gene Wengert, technical advisor Sawing and Drying Forum:
As background, 22% MC is exceptionally wet and is equivalent to exposing wood to 93% RH roughly. I wonder if you are a bit drier, especially if the wood has been stacked for a week or more. When gluing the surfaces together, you are concerned about the surface MC and not the average. Did you use a pin-type moisture meter and then stick the pins only in the surface (outer 1/16")? Maybe you can use a "hair dryer" to dry the mortise and tenons a bit before gluing, and do not glue them hot.
I would use a PUR (polyurethane) adhesive and would talk to the manufacturer to find out which one (within the family of PUR adhesives that a manufacturer makes) is correct for you. Gorilla is just one and there are many within the family; some might be better than others for your use. Because of the sensitivity of epoxy to UV degradation, you might be wise to avoid epoxy.
From the original questioner:
Thanks for the responses. I did use the short pins - 3/8" long. Typically I'll make an end cut and get into the center of the board, but I just did this on the surfaces for a quick read. I tend to believe the 16% more than the 22%, but that may just be what I want to believe rather than fact. I hadn't thought of the UV epoxy thing. Would that matter with this all being big mortise and tenon? I think of this Cedar as moving a bit from today to three years from now, so a more flexible bond may be more desirable than a rigid one. Epoxy is to the rigid side, PUR on the flexible side - correct?
From contributor B:
We did a pergola last year with red cedar and used PL brand premium polyurethane adhesive for the 6x6 half lap joints in the 30' main beam. I also did a test joint and tried to get it apart with a sledgehammer, and failed. The joints are still tight and sound. This stuff has gained my confidence in many other uses as the strongest adhesive around. Note that while it comes in a caulking gun tube, it is not anything like generic construction adhesive - it is much tackier and tenacious in its grab to dissimilar surfaces like wood, metal, and concrete. I would definitely use it for the project you describe.
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