how can you clean some of the old white corosion out of your radiator after it has been sittin for a long while?

You can buy a flush and fill kit and flush it with water or you can use a chemical flush (if you make sure you follow the instructions to the letter). In the end, though, you are probably better off replacing the radiator.

Chances are, if your radiator is corroded, other parts of your cooling system are also corroded. Make sure you check the whole system, especially the thermostat and the water pump. You might also check the heater core since it is similar to the radiator.

Is it brass? Those you can take to a radiator shop. If it’s aluminum/plastic, I’d go to Autozone and get a replacement.