It’s easy to round over the corners you want to stay crisp when you’re hand sanding a narrow edge with a sanding block. Here’s how to avoid it. I support the sanding block with a square-edged piece of scrap underneath it, and I move the scrap back and forth with the block as I sand. The scrap serves as an edge guide against the workpiece to keep the sanding block square to the surface I’m sanding. When the sandpaper dulls or loads up, simply reposition the scrap guide under the sanding block to expose fresh sandpaper.
– Charles Mak