The Guardian I knew that my wife was about to go into labour because she fetched her toothbrush - not for her hospital bag, but for cleaning behind the radiators. Then I realised that after a woman has been through labour, she never, ever forgets a nesting-based promise, So there I was trying to remember the precise way to paint radiators. The old column ones need to be done from the inside out using a flag brush - a brush on a wire, that you can bend to shape. Push the brush inside and start at the back.
Standard radiators are a different story.
You need to tackle the vertical recessed sections first, then the raised panels or ribs, and then the awkward side against the wall. You could always put up a radiator shelf, so you can forget about the behind bit altogether. Even better, box in the entire radiator with a grille and never paint the thing again. But that's being defeatist. For the great radiator finish, use one of those curved brushes, called a bent fitch .