Walnut Glue-Up Adhesive Failure
From Jeff Pitcher, forum technical advisor:
I suspect that you have a few issues. First, you probably aren't getting the appropriate pressure if you're trying to glue 8/4 lumber. Next, it's probably very dry there which might not provide enough moisture to kick over the glue you're using. Finally, I'd suggest using a PVA glue as an alternative. The PUR is overkill for your project.
From contributor M:
Contributor E and Jeff make valid points.
Other things to look at:
What is moisture content of lumber?
Is stock flat and straight and freshly machined?
Do you have proper clamp pressure across the lamination?
Did you wait 24 hours after gluing to allow glue to get to full strength before machining?
When using Gorilla glue on kiln dried stock, we wiped both surfaces with a wet cloth prior to gluing. Titebond is probably a better choice if using for interior use.
Glue joint is typically stronger than wood. Is the failed joint only on the glue line or is there wood tearout as well?
From contributor R:
I'm not sure you can say that Gorilla glue is an efficient gap filler. It fills it with foam, but that has limited strength. Epoxy is the best adhesive that really retains strength in gaps. I agree with all the other thoughts.
From contributor C:
All are good points above. Of utmost importance is the flat fit of the planks. They must be jointed so there are no gaps prior to clamping. The slight ridges and lines from planer and jointer knife nicks must be scraped out so the glue has the best possible flat fitted surfaces to adhere together. Titebond II for dark woods will give excellent results. Flat relaxed fitted joints without the need for excessive clamp pressure will give the best long term results. You cannot hope for the glue to hold warps and gaps closed. They will be glue starved when you clamp things tight enough to close the gaps.
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