Wire-brush the rust off first, and prime and paint using a rust-resistant paint. You can get advice on paints from the stores that sell them.

A brief google search shows a lot of sites with advice on this, many of them in the UK for some reason.

Here is a link that might be useful: A

part of it depends on how hot the water is- I've used regular house paint on my baseboard covers, but went with heat-tolerant spray paint for the radiators upstairs- better safe than sorry, eh?

a wire brush will take flakes off- a small, fine-gauge one on a drill will get into the details. there are also brush-on solutions that will 'stop' the rust (convert it chemically into something stable) that can be painted over- this is a good solution if you're not working with an ornate victorian antique (and honestly, sometimes the ornate ones look better, well, less ornate- and it will dampen any detailing in the castings)

I've restored several radiators. First, wire brush it, then strip it as much as possible with paint stripping solution. This is a messy and long task, but it does eventually get most of the paint and rust off. Sand well, then prime and paint with a paint made for metal. With this method I've had no problem ( 8 years and counting) with rust returning and the radiators look "like new."