I just go with the bare minimum grit to get the job done, If your doing that then your doing the best you can. A steamer can help save some diameter also. You have to break the shiny areas left in the middle by hand with a folded piece of paper or something, and without removing too much material, because you need as much wood around it as possible to expand and fill the dent.
Unsealed shafts can be a pain to even get reasonably clean, and rarely clean up perfectly. I don't use bleach, but sometimes a cleaner that pushes the dirt to the surface works well. I quit using scotch-brites along time ago. I do use the magic erasers sometimes though. I use to use a few things on My own shafts that cleaned well, but I would never use them on a customer shaft, because they envolved wetting the shaft more then I would take the risk on with someone else's shaft. My personal shafts don't get that dirty anymore, because I keep them sealed well from the get go. The first time you work on a customer's shaft is usually the worst.
I also seal every shaft really good after I work on It. If the player does'nt like the buffed sealer then I knock the top off for them to give them the feel they like. This not also seals their shaft well, but saves me alot of work the next time I have to work on that shaft. Sometimes the sealer helps fill the ferrule issue Where it meets the wood, but not in the really extreme cases.
Trimming the tip, I have found that using the blade too soon causes the knife to be too unstable and jumpy, but if you bite down too hard and hold It tighter, It can tear some tips up. or after trimming with the tool too close to the ferrule, It's hard to get a good bite on the edge of the tip, and can make the utility knife scratch the ferrule just enough to require sanding, so what I do is take the tip down with the tool, but leaving a decent lip there still to get a bite on, and It cuts alot smoother requiring very minimal sanding, unless the ferrule Is pitted bad to begin with.
Some shafts are so bad, you just do the best you can, and save what diameter you can. I know too well that they are time consuming & hardly seem worth the time, effort, and little bit of money you get for It when you get a shaft in like that, but If they end up being a regular customer, It will pay off later down the road. I get them quite a bit, and even though I'm not very impressed with how clean I got them all the time, the customers still seem overwelmed by the difference, and bring their work to me. They also tell people they know about you. I get lucky sometimes, and a shaft that looked really bad, actually cleans up easy for me.
Greg