Overheating during idle

If you have access to the front of the rad, get it up to operating temp, thermostat open,, and just touch the front of the rad with the back of your knuckles, about half way down,, all across the rad.. What you're feeling for is cold (cool) areas, which will indicate plugged rad tubes.. If it's hot all the way across, which I doubt considering the rusty water, the rad isn't likely the prob... Laser temp gun is handy here, if you got one..

The bigger the cold area = bigger lost cooling capacity..

hope it helps..