“I hate the potential mess that accompanies poly glues, but once you’re used to handling them they’re darned good if you want bomb proof construction or maybe something with gap filling capabilities.”  <- mention of gap filling…

Polyurethene glues need moisture to cure, “air curing” is a side-effect of that as the air is supplying the moisture…

At any rate, the foam has no structural strength (near zero shear strength, which is why it is so easy to scrape away) and should not be relied upon.  A tight fitting joint with just a thin-film of glue will always be best, no mater what type of glue is used.

If there is a gap to be filled and it cannot be addressed by re-working the joint line then an epoxy is probably a better choice.  However, just any old epoxy may NOT be the best choice as they are widely varied in their adhesion properties and modulus of elasticity.  Translation, some are more sticky than others and some flex while others are brittle.

VERY much a generalization, but the shorter the open time (i.e. 5 minute vs 60 minute formulations), the more flexible it epoxy remains.   This is good in that it lets you control aspects like slumping and allow for some flex in a joint.  This is also bad because it allows for slumping and some flex in a joint.

You can do crazy stuff to glues and epoxies by messing about with the atmospheric pressure, we’ve made good, high-end West Systems epoxy turn to a jelly-like goo just by the application of pressure.  A non-woodworking experiment where we needed to pot some equipment to be placed about 1km down a water filled hole which would later freeze solid.    The steel test vessel was nicknamed “the bomb” due to various behaviors exhibited during pressure tests. 🙂

We now return you to your regularly scheduled tangent on gluing for woodworking.