Over-tightening and its telltale signs
Cranking a clamp past what the joint needs doesn’t make the joint stronger — past a certain point it just crushes wood fibers under the pad, leaving a visible dent or crescent-shaped mark once the clamp comes off. If you’re seeing the workpiece visibly compress or the clamp handle suddenly gets much harder to turn near the end, you’ve likely already gone past the pressure the joint needed — see our clamping pressure guide for the actual PSI ranges glue manufacturers recommend.
Skipping cauls/pads — what actually happens to the wood
A bare metal clamp jaw against bare wood concentrates all the clamping force into whatever small contact area the jaw touches, which is exactly what causes dents and, on some woods, dark stains where steel jaws react with the wood’s natural tannins under pressure and moisture from glue squeeze-out. A wood caul (a scrap board placed between the jaw and the workpiece) spreads that same clamping force over a much larger area, dropping the actual PSI at any one point and protecting the surface. Non-marring pads — like the ones on the Bessey GSCC4PK-C budget bar clamp set — do the same job built into the clamp itself, but a caul is still worth adding on show surfaces.
Uneven pressure across multiple clamps
When you’re using several clamps across one glue-up, tightening them one at a time to full pressure before moving to the next can walk the joint out of alignment — the first clamp fully seated can create a pivot point that the next clamp then has to fight against instead of just adding even pressure. Snugging all clamps to light pressure first, checking alignment, then bringing each up to full pressure in a second pass keeps the joint from shifting as you go.