Some large knives won't even heat up that much on a belt grinder. I keep a container of water handy and dunk the blade as needed to keep it cool. Keep the blade moving and pay close attention. Some of the small stainless knives will need to be cooled between every pass. Do serious stock removal with the coarser grits. They don't heat up the blade as fast. You are moving the blade faster with those, too, because they remove a lot of steel in a hurry. My grinder only has one speed, so I have adapted my methods to that. Otherwise, I'd slow the belt speed a little.