fine sandpaper will work, but there is usually more than rust there, old grease, dust, dirt, problem with sanding it, it will work great, right after, but later it will give you issues again, when contacts rusts, the rust eats metal, and makes it thinner, and not as flexible\ springy anymore, sometimes tabs just break off during cleaning, we usually replace sockets, they aren't expensive, for older models, a brand "help" makes replacement sockets. wire clip splicers make the whole procedure a 30 sec thing. for more recent cars, you need to see a dealer, but i have not seem many newer cars that needed sockets cleaned\replaced, mostly after collision.