I was using way too much paste for my endpapers for years
I was binding a custom journal for a friend last week and the endpaper just would not lie flat after drying, it kept bubbling up. My mentor from my apprenticeship, Frank, happened to stop by the shop. He took one look, poked at the paper, and said, 'Kid, you're drowning it. You only need a whisper-thin layer, like you're barely touching the brush to the glue.' I'd been applying it like I was buttering toast. I tried his method on a fresh sheet, using maybe a quarter of the paste I normally would, and it adhered perfectly with no wrinkles. Has anyone else had a specific tool or technique tip that completely fixed a stubborn problem like that?