Prepare
Cold, clean, primed
Turn the heating off the day before: never paint a warm radiator. Dust between the sections with a brush and vacuum, wash with degreaser, rinse. Wire-brush or steel-wool any rust spots back to sound metal, then sand the old paint with 180 grit to give it a key.
On bare metal, brushed rust spots and new pipework, apply our Multi-Surface Primer; on sound old paint it is only needed where you have gone back to metal. Protect the wall behind with a sheet of card slid between radiator and wall, and the floor with a dust sheet.