Add an escutcheon plate behind the knob to better protect the enamel finish. These plates are available in various sizes, from hardware stores, home centers, woodworkers' supply stores.
Another option would be to use something called ‘fender washers’. They are washers with a small hole in the center as opposed to the larger holes in a conventional washer. They are available from home improvement centers, Amazon, eBay, etc.
Your fingernails are chipping it. Try to grab with your knuckles. It will protect your nails too