What is 'feather-edging' and why avoid it?

Feather-edging is the practice of troweling a repair mortar out to a super-thin layer (a feather edge) to blend it with the surrounding surface. This is bad practice in structural repair because the thin edge dries too fast, cracks, and delaminates. We always saw-cut the edges of a repair area to a minimum depth (e.g., 10mm) to provide a square shoulder that the repair material can butt against for a durable bond.