There is a sharpening system known as scary sharp. It typically involves adhering sand paper down to glass plates. Start at a high grit, and sharpen your tool down through the grits. It is perfectly valid, and can give you a great edge. The only issue is the cost of sandpaper adds up.
If you only have a few tools to sharpen, it works great. Sometimes you want to clean up a really rough ebay tool, and don’t want to use a nice diamond stone on a rusty hulk. Flattening water stones is rough work, and best done with disposable sand paper. They can help flatten issues on cast iron tables. Basically lots of good uses.
I have some decent diamond stones, but still wanted some glass plates to do occasional sharpening and clean up with sand paper. I went to a local glass company and told them I wanted 1/4″ 4×10″ float glass for this purpose. I ended up paying 50 bucks for 9 plates. The edges are a little rough, but not sharp. Just not pretty. They even put nice little square foam pads as feet.
I am tickled pink at how nice and affordable these were. I don’t plan on using them a lot, but at the price I got how could I not go for a pile? This is probably a stash beyond life expectancy! I would urge woodworkers and tool users that need to sharpen flat objects to go to their local glass shop and see what they can do. A little super 77 spray adhesive to stick the paper down, and a sharpie, and you are in business.