Most of the blades I have seen, are benefited by the use of an angle grinder. I imagine a very coarse belt would work well. Even after that kind of aggressive grinding, they usually are not nice flat "new" sharp on the bottom of the blade. One small engine repair and sales shop I dealt with told me, they used an angle grinder and finished with an abrasive flap wheel on the angle grinder. He said the last step made the blade very impressively sharp and nice looking, however, that sharpness very quickly disappeared when put to use on the mower. It did impress the customers. His Neary lawn mower blade grinder, sat there gathering dust.