Update: my 1890s ledger split down the spine and I made it worse before I made it better
I was in my basement shop last Tuesday rebinding a customer's 1890s ledger when the spine split right down the middle, and I panicked and grabbed the wrong paste. It was a hot wheat paste mix I'd used out of habit, and it soaked into that old, dry leather and left a dark stain about three inches long. I set the book down, walked away for an hour, then came back and used a dry cloth and a soft brush to pull off as much as I could. It's still a little darker there, but I got the joint lined up and sewn back on a new cord after two more tries. I charged the guy $40 instead of $120 and he was fine with it. Has anyone else ever made a stain worse trying to fix a split joint? What do you keep on the bench for old leather that won't leave a mark?