Well, it's hard to set hard and fast rules of when to drop below a 1k, it's an experience judgement. When I use my 600 on razors I set the bevel, kill the edge, and set it again with the 1k. If it's worth that much time and steel, then it's worth it. If you're just doing one or two razors you could use 400 grit W/D. For me, I'd replace my 600 in a second. It's a nice knife stone too!