How should I remove corrosion like this from radiator connections
You can clean the threads with a soft wire brush without being too aggressive and clean the tapered area very carefully with a very fine steel wool (not sandpaper), so as not to score the taper and cause a leak that can't be stopped. Make sure to clean both pieces, the one on the radiator and the one in the picture.  If the tapered area is rough or deteriorated in any way, you could coat the tapered area with a thin coating of RTV silicone and tighten the union nut.Do not use teflon tape on the tapered area, use silicone only.