Hey everyone...first post here so, hello. I spent the day yesterday sharpening all my plane blades and having to work really hard to get them flat. Usually this isnt a problem so I was a bit confused. However, I noticed near the end of my sharpening that my flattening stone was worn concave. I'm using the "Norton flattening stone for waterstones". Do any of you know the best way to true up this stone so that I can go back to having flat waterstones? I can't find a glass store around to get plate glass. Thanks!