Thermocast sink

We bought our house new about 4.5 years ago. All the appliances are black, but the kitchen sink was the shallow contractor grade steel one. So we bought a nice Thermocast black sink from Home Depot. Within a few weeks the glossy shine was gone on the interior portion of the sink. The upper surfaces are still pretty but the inside is dull, and looks like the polish has rubbed off. Anybody know what I can do to remedy this?