I picked up some radiator enamel paint at the weekend to tackle the same problem in my bathroom. Also got a few scratched rads arpind the house that could do with touching up.
If you are painting a radiator for protection, you can do a much better job by taking it off the wall first. Then you can rub it down lightly with wire wool; apply Kurust on any rusty patches after rubbing off the surface; apply an anti-corrosive primer before your topcoat.
It is impossible to paint the bottom and back of a radiator when it is fixed.
Also, I presume that if your rad is getting rusty, it must be cold enough to get condensation on it (or does someone hang wet towels on it?)
Have a look at the Wiki/building/condensation thread and see if you can improve anything there.
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