Originally Posted by
Tony Shea
I honestly think using sandpaper on float glass/granite surface plate still might be the best way to flatten backs. I have tried just about every method there is, diamond stone, kannaban plate with loose silicone carbide, kannaban with loose diamonds, sandpaper, etc. Flattening blade backs is hard work and the pressure you apply is not what diamond stones are designed for. This heavy pressure and intense back and forth motion quickly pulls the diamonds off the surface of the plate. I will say that loose diamonds on a flat steel plate works well but eventually dishes the steel plate over time. Eventually after all this experimentation I've gone back to sandpaper stuck to plate glass. The longer the plate glass the better as you are changing sandpaper much less often. I also like having a piece of wood help put pressure down instead of just my hands as it helps apply pressure evenly and doesn't cramp my hands. I typically start with 120 grit and see how quickly that works and if absolutely necessary move to 80 if there's too much for the 120. My goal in flattening blade backs is efficiency, I hate this process and want it to be done as fast as possible. Therefore I believe the fastest and cheapest method is sandpaper on plate glass.
I'm not sure where he's at these days but David Weaver used to be a great resource on this very subject. He has a YouTube channel now and believe there might be some videos of this process on there as well as some old threads here at Sawmill. Unfortunately he left Sawmill due to some personal reasons and has been missed ever since. Everything I know about flattening blades is certainly a spin off or exact copy of his guidance. He also has some great information on plane making, sharpening, planing, etc....