I'd advise strongly against a chemical paint stripper because the cleanup would be difficult, and residue could cause the new paint to fail if it is not cleaned off adequately. Chipping or scraping is good. A wire brush can reach some places. I have stripped a few rads by chipping with a screwdriver and hammer. Just chip an hour a day, and in a few weeks that paint will be gone. ;-)
Use a rust-inhibitive spray primer, and a high temp. top coat.
Any color is fine, most were originally painted with a thin metallic paint, gold or silver, but you can do whatever you want; the heat output will not change noticeably.
Casey

I have always just painted the radiators w/ a semi gloss that was same color as the trim in the room. I once tried to strip one w/ a wire wheel on a drill and all that did was dig into the radiator itself. Since then, I have not had a need to strip any. Just be sure to paint them when they're cool and clean first w/ a TSP and water mix.

Nice thing about buying an unmaintained old house is there wasn't much paint on the rads. I wirebrushed well and used gold metalic paint on all which looks 'right' to my eye in an old house. The early paints (25 years ago) tarnished quickly. The past few years I used Rustoleum spray paint which holds up very well. It comes in a few different shades of gold metalic (among other colors).

The only inappropriate paint on a radiator is metallic paint if it has a reflective finish and a high content of a metal like aluminum or colored aluminum. Such finishes were not used in older houses because they were know to be poor transmitters of radiant energy (infrared energy that is transmitted directly to objects in the room without warming the air) and radiant energy is very important in a house with many radiators placed away from windows.

A bright shiny true aluminum surface would, of course, be considered a "radiant barrier", only emitting about 4% of the energy. A shiny high content aluminum paint might emit 40% to 60%. All other paints or even a bare radiator would emit about 94%.

The only reason I can think of to use a shiny metallic paint is to reduce the efficiency of the radiator because it is over-sized for the room it serves. But since an inefficient radiator just returns water to the boiler at a higher temperature, it would probably be more energy efficient to maintain the radiator's highest output and lower the temperature of the circulating water.

Otherwise, you can paint your radiators any non-metallic color or finish you want because the emittance would be the same. Mine were originally dark green in 1891 and only a few have been repainted.

Use whatever paint will adhere well and will be easy to clean.

I hated the idea of having to do up my radiators myself so last fall I searched for contractors who did designer radiators online and engaged one to install ready made and painted ones into my new house. Never regretted it because I have heard of the hassle it can be to DIY.