So I’m attempting to level the frets on some of my guitars, but I’m having some trouble with a couple things. I have all the correct tools to level, crown, and polish the frets, but I’m getting stuck on the leveling part. I’m following Ron Kirn’s guide, and I’m using a marble threshold from Home Depot with some 320 grit sticky back sandpaper stuck to the marble part of it. I tried using his method to get the neck straight before sanding the frets, but on both of the guitars I’ve tried it on, there’s always a couple frets in the middle that don’t get touched, no matter how I adjust the truss rod. After getting it as straight as I could, I started leveling the two guitar necks, and even after a solid 30 minutes of sanding all the frets like Ron says to do, there’s still frets in the middle that won’t get touched. I don’t think it should take this long to do it. Could my marble threshold not be level? I checked it with a straightedge and it seemed pretty level to me. Any help would be appreciated.