Warning: my old boss at the bindery in Portland told me to stop gluing up spines with PVA straight from the jug
He said to thin it a hair with water and work it in warm, and I blew him off for about 2 years because it felt like fussy old-timer stuff. Last month I did a batch of 12 case bindings that way on a whim, and the spines opened flat with zero crackle, while the straight-glue ones from the same run are already stiff. Now I keep a little squeeze bottle of thinned glue on the bench and I'm not going back. But a guy at my local guild says thinning weakens the bond over time and you should just use fresh glue and move fast. So which is it, thin it or leave it alone?