Why the strongest glue joint is the thinnest one

Why using more adhesive produces a weaker bond.

A glue joint squeezed down to a thin, even film reaches its full strength with almost nothing wasted, while the same glue left thick and generous between the two surfaces ends up weaker, because a thick layer behaves like the soft bulk adhesive it actually is rather than like a joint constrained tightly between two rigid surfaces.

What is actually happening

Squeezed thin between two surfaces, a layer of adhesive cannot move or deform without dragging along whatever it is bonded to on both sides, since there is almost nowhere for it to flow, and this constraint is what lets it resist shear and tension loads so effectively, spreading stress evenly across the whole bonded area rather than concentrating it anywhere. A thick layer has none of that constraint. Deep inside a generous blob of glue, material sitting well away from either surface is free to deform much the way the same adhesive would deform sitting in a pot, and a joint loaded through a thick layer tends to concentrate stress at its edges rather than sharing it evenly, precisely where a crack is most likely to start. Thick layers also cure less reliably, since a moisture-cured adhesive like cyanoacrylate depends on humidity diffusing in from the surrounding air, and material buried well below the surface of a thick bead may be starved of that moisture for a long time, or a bulk-mixed epoxy may trap air bubbles that a thin, well-spread film would never hold in the first place. A trapped bubble is worse than it sounds, since it is not simply a small gap doing nothing, it is a sharp discontinuity inside the glue line where stress concentrates exactly the way it does around any sudden change in shape, giving a crack a ready-made place to start well before the surrounding, properly cured glue would have failed on its own.

The syrup comparison

Squeeze a spoonful of honey or golden syrup between two flat plates and try to slide them sideways past each other, and the result depends enormously on how thin the syrup has been squeezed. Pressed into a thin film only a fraction of a millimetre deep, the plates resist sliding strongly, because the syrup has almost no room to flow and has to shear across that whole tiny gap all at once to let the plates move at all. Leave a thick pool of the same syrup between the plates instead and they slide past each other far more easily, since the syrup in the middle of a thick pool can simply flow out of the way rather than resisting the motion, the same distinction between a constrained thin film and an unconstrained thick one that decides whether a glued joint is strong or merely thickly covered.

One figure worth keeping in mind

A well-clamped structural adhesive joint can reach its full working strength with a bond line only a few hundredths of a millimetre thick, and packing in ten or twenty times that amount of glue does not multiply the strength by ten or twenty, it typically weakens the joint, since all that extra thickness is added exactly where the adhesive is least constrained and least reliably cured.

What follows from this

Because thickness works against strength rather than for it, the instinct to apply a generous amount of glue for a stronger bond is usually backwards, and clamping a joint firmly enough to squeeze out the excess, leaving only a thin, even film behind, is what actually produces the strongest result. A joint with visible gaps or unevenly spaced surfaces, which forces a thick glue line to bridge the difference, will always be weaker at those thick spots than a well-fitted joint with a uniformly thin bond line, regardless of how much adhesive is used to fill the gap. This is also why a well-made joint depends on how accurately the two parts were shaped to fit each other before any glue was involved at all, since no adhesive, however strong, can fully make up for a gap that should never have been there in the first place.