Originally Posted by Bob Carnie
I've removed a hair or two and some rather long scratches using the knifing/etching technique Ian describes. I commonly use etching to remove black spots on the print. It takes some practice, but, once mastered is reliable and much less intrusive than bleaching, which spreads into the surrounding emulsion and requires a lot of retouching.
I use a surgeon's scalpel and a 8x or 10x loupe. The trick is to remove as little of the emulsion as possible, scratching down through it instead of gouging it out. If you are lucky, you can scrape away enough of the dark area to hide the defect and still leave a little gelatin over the baryta layer.
If you aren't so lucky, you'll scrape the emulsion off completely, leaving the white baryta layer, which then needs to be spotted back up. I use spotone with varying amounts of gum Arabic added to match the sheen of the print surface.
Practice on a few scrap prints and you'll get the hang of it. After a while, you'll be etching regularly.
Some advice: take frequent breaks from your work and don't drink too much coffee before etching, otherwise your hands will be shakier than you want.