Gluing Wet Cedar for Exterior
Advice on glue choice for outdoor architectural designs built with Cedar. August 6, 2008
We have some large gates to build with some Western Red Cedar that is between 16% to 22% MC. They will be draw bored mortise and tenon, and my first thought was West Epoxy. Then I considered urethane (Gorilla) since it likes the moisture. Now I'm divided and curious as to what others with direct experience will say.
From contributor A:
Use the Gorilla glue, but use it sparingly so you don't have tons of foam to clean up. The West System will boil out if it is too wet, it doesn't like water that much until it's cured.
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.