Heat a oven at convection bake to 175°. Blot grapefruit with paper towels and place on racks on a baking sheet. Cook grapefruit rounds at 175° (convection bake) for 4 hours or until dried to the touch. They shouldn't be sticky at all, but crisp. If your oven doesn't have a convection option, bake at 175°, but cook time will be longer.