It’s not easy sharpening a curved edge with a flat stone, so why bother? Get a curved stone like one of these slipstones from Grizzly. These tapered Japanese stones can sharpen gouges from 1/2″ to 2″ wide. Made from aluminum oxide and available in 240, 1,000, and 4,000 grits, these water stones both sharpen and polish.
One curved slipstone will run you about $16.