Use a brush, You can buy Radiator brushes for the old style of Radiator if required. Don't bother taking them off the wall, just paint what you can see, or use Radiator covers to hide them.
Here's a post that might help - https://www.bobvila.com/articles/how-to-paint-a-radiator/
Hi Deb, depending on the look you want you could paint as per John's advice using a brush. You might consider making it blend in with your walls or another idea is to make it a very striking color as a conversation piece if you wish.
Downstairs we have radiator covers, but upstairs we do not. Years ago I painted the radiator in my bedroom the pink color I had used for rest of room. For years I never turned that radiator on, but lately I have been and the paint is peeling. If you do use covers, you can add reflective metal fastened behind the radiator to "radiate" more heat because covers do block some of the heat into the room
Be sure to use a paint meant for metal and heat. Here's a link with more information:
They do make paint for radiators specifically..... https://www.google.com/search?client=opera&q=radiator+paint+colors&sourceid=opera&ie=UTF-8&oe=UTF-8
Hi Beb Hope this helps