If you are going to use them to grind steel no preparation is needed BUT unless you apply a belt grease or polishing compound regularly as you go they plug off and cutting will decrease . If you are going for a fine cutting - polishing belt to use with say green chrome, buy one that is doesn't have any grit. otherwise go for a say a 600-800. Anything else has a tendency to not finish as you were hoping for. as you may have read, you must get that belt down to near the canvas, or whatever backing it has but still try to end up with an even finish. The edge of a worn out file will work. Yes, that can be very messy work. Clean freaks really hate it but I know one that will still go through the preparation part because he enjoys what the belt will do afterwards with the polish. This is a little on the "what's going on" thing? Usually the belt isn't prepared enough or you aren't applying enough polish. It needs to be loaded.
It can work very well. I went back to trying this probably 6 times before I got it to work for me.The last time was when I had a maker take me through what had to be done, that used this on every knife he made. Frank
