As for why the "horse in a bottle" type instant hide glues just cannot ever be as strong as pure pearl hide glue.
If memory serves, the answer lay in the fact that in order to keep this stuff viscose and mould free whilst it sits cold on a shelf, for months at a time, manufacturer must add preservative and retardants like urea or some spirit based thinners. These retardants must cause some kind of permanent dilution in bond strength. I would never use this stuff on anything but paper to paper joints.
Real Hide Glue is good, soak it, heat it, use it quick. If your pushed for time, use it in a hot room with pre-heated parts. You can also try blowing the newly clamped area with a heat gun.
Why rumour has it, in order to achieve a perfect glue joint, some early luthiers would glue and clamp the bracing and then burn their viols scrapping back the charred outer to shape them. I don't know how true that is but it sounds good enough to tell the kids and get away with.
What ever you do, when working with hide glue I think the key word to success is METHODICAL.Get your bits and pieces ready, your clamps, cauls and what not all at hand. Turn off your phone, put "Do Not Disturb" on the shed door, and whilst your glue comes up to temp, clear your mind for the task ahead. Lay out your work at the ready and if you are unfamiliar, do a dry run first.
My own experience only extends to instrument repairs, some pretty major, but I have not built yet. That being said, I have used hide glue for other applications and IMO it's great stuff. One big plus, once it sets-up, it continues to tighten it's bond after the clamps are removed as more moisture is expelled making it perfect for repairs in tight places.
Frank Ford explains the benefits well at
Frets.com.
Cheers All
Kim
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