Reply by Madmark2

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Posted on Short glue test

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2655 posts in 1670 days

#1 posted 02-28-2021 08:28 AM

PVA glue dries as the water in the glue is absorbed into the wood. It does not out-gas or depend on in or out flow from the joint. Thus the drying time is the same over the entire surface. Most strength variations are due to coverage gaps more than anything else.

Glue starvation by over clamping is a myth. Again, these failures are generally coverage related. Clamping pressure should close the gaps but not crush. Use blocks to spread the force and cauls to keep everything flat. If you have to use pipe clamps to close the gaps your joinery is sloppy.

Under clamping can lead to weakness as not all glues are “gap filling”. This is why “rub joints” aren’t usually as strong as a clamped joint of the same dimensions. Rub joints are sufficient for lots of non-load members, trim, etc. applications.

Clamping pressure fans out at a 45° angle from the clamp location. If the joint is in the middle, the clamps should start 1/2 the width in from the ends and be spaced about the glueup width apart.

Other glues may harden differently. For example RTV adhesive out-gasses acetic acid (vinegar) fumes as it cures. Some epoxies out-gas who knows what.

Direct comparisons are difficult esp. if the manufacturers instructions aren’t followed to a “T”.

-- The hump with the stump and the pump!

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