The easiest way to fix a damaged radiator valve (if cleaning it does not work) is to swap it out for a new one. Learning how to remove and replace your old valve is not too complicated. Before starting, make sure the water in the radiator is cold the entire time that you work to prevent burn injuries.
Turn off the water source for the radiator (usually a valve just below the boiler). Also turn off the heat in the boiler. Allow enough time for remaining water inside the radiator to cool so that you prevent burns from steam and hot water. Never open a radiator that is not completely cool.
Place the rags around the radiator. Place the pan under each location as you work on it. Wear safety goggles and beware of steam or hot water that may be under pressure.
Drain the radiator. At the bottom corner of the radiator on the lower end of the pipe is a pipe that has a square in it. This square opens the radiator to drain the pressure and some of the water/steam. Next to this square is a hollow, empty pipe that points downward. Slip the garden hose (after testing it for leaks) over the end of the pipe and tighten it onto the pipe with a hose clamp.
Lead the hose along the floor and out the door to a point where the hot water can safely drain (away from anything alive). Make sure that there are no kinks or blockages in the hose. Open the pipe slowly by turning the square where the hose is attached to drain off water and steam. Listen for the hissing sound as the pressure is released into the hose as you turn the square with the channel lock wrench.
Close all valves you had open on the radiator with the radiator still turned off. Remove the hose and place rags under this spot to catch any drips.
Where the pipes enter the floor is a valve. This valve has a square at the top but controls the flow of the water into the radiator. Feel the pipe around this valve for heat to make sure that the pipes are cold before continuing to work. Replace this valve by looking at the pipe where it first goes into the radiator. There should be a ring around the place where the pipe from the valve joins the radiator. Grasp this ring in the adjustable wrench and turn it counterclockwise to loosen it until it completely separates from the radiator. Do the same thing on the ring around the pipe where it goes into the floor to loosen the ring. Remove the valve.
Use the channel lock wrench to remove the small section of pipe that goes into the radiator and replace it with the new one that comes with the new radiator valve kit. You may need to use what is called a spud wrench to remove this section of pipe. Use steel wool to clean the old metal pipe where the rings were connecting the old valve and add three layers of Teflon tape wrapped around the exposed threads on the end of the short adaptor pipe.
Put in the new valve in place of the old. The new valve should by a safety valve that has a thermostatic regulator on it. Add a strip of Teflon tape wrapped around the pipe threads three times at both locations where the rings will be screwed on. Attach the new valve rings that hold the new valve in place by slipping the rings toward the pipes. Hand tighten both rings and then use an adjustable wrench to twist the new rings firmly into place.
Restart the water in the system. Twist the square at the top of the radiator so that the radiator can let the air hiss out of the system. When water starts to come out, turn the square the opposite way to close the system (remember that it is righty tighty, lefty loosey). Turn on the heat for the boiler again.
Add sealant to all pipe joints to help prevent leaks. The Teflon tape can work well alone, but the sealant adds an extra layer of leak protection. Replace the old valve with a thermostat/thermostatic regulator for tighter control of the heat from the radiator.
If you have a conventional cylinder-tank system, get help from a professional to turn off the heat and pressure in your system or to restart/refill your system to avoid harm that can come as a result of doing this part yourself. All radiators should have a radiator cover so that living things do not come into direct contact with the hot radiator. Always be alert to the fact that hot steam and water can spray out of any valve. Always keep yourself away from the openings of the radiator where steam and hot water can escape.