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.
This is how one reclaim company paints radiators. http://www.rbsoak.co.uk/cast-iron-radiators.html
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
http://www.art-restoration.com/press6.html : Years ago, when the gilding on antique mirror and picture frames became dull or darkened, the handyman of the family would visit the hardware store to buy a can of his favorite radiator paint. The choices for the “restoration” were silver or bronze. The radiator paint was great stuff. It wore well, covered the original gilding and didn’t chip. Well, here we are at the start of the twenty-first century, and those antique frames with their radiator paint finishes intact are still coming through the door of the New Orleans Conservation Guild.
you can buy J-B Weld for about five bucks at most automotive and hardware retail stores.