paint with aluminum or bronze radiator paint, if you can find it. These paints reduce the efficiency of the radiators, but I have never seen such paint peel. If you use any paint, thin coats are most important.
The other way: Scrape and peel off all paint that will come off, then spray on an HHR paint that comes in an aerosol can. The HHR stands for high heat resistant, which is probably necessary with the high heat of steam radiators. It is not needed for hot water system radiators.
Do not use latex paint on a warm surface.
The latex paint and primer could develop rust spots, and latex paints may not hold their color on a hot radiator. It is better to use an oil primer (Kilz is good), and an oil-based finish paint. Actually, you could use an aerosol spray enamel as a finish coat. Of course, be careful when spraying indoors. With care and practice it is possible not to get the spray paint all over the floor, walls, and anywhere else you don't want it.
A reclaim company paints radiators. One of the radiators is finished in red oxide primer, while the other two are finished in a clear powder coated lacquer.
Effect of Paint : http://www.jcgbs.com/users/jft/Radiator%20Color.htm