How do you sharpen a card or cabinet scraper?
First make sure the scraper's edges are flat, square, and sharp, using a stone such as a diamond stone. Then use a hard steel burnishing rod, angled slightly, to push and deform the edge into a small hook or burr; a bigger burr scrapes more aggressively and coarser, while a smaller burr gives a finer cut. The burr can be rolled back down and re-burnished four or five times before a full re-sharpening is needed.