I wouldn't sand it. Clean your frame as much as possible without removing metal around the area where the stars and numbers are suspected. Get some duct tape and tape around the area enough to lay a towel soaked with evaporust and cover it for a few days. You need to let it have some time to work, patience is your best virtue. Use a small stainless wire brush about the size of a toothbrush, I buy a handful everytime I step in Harbor Freight Tools. It might require several brushings and recovering with the evaporust but eventually you will end up with a rust free area.