I've done a few radiators in our house over the past few years and each time, I used an aerosol can of radiator paint - more than happy with end result every time.
I can't remember the brand, but I'm sure there'll be plenty of decent ones out there.
Preparation is the key and put plenty of newspaper behind the radiator, so when you're spraying the edge, it goes on the newspaper and not all over your wallpaper .... unless you're redecorating, in which case it won't really matter.
Just a quick light spray, then give it a little time to dry, then reapply - don't over-spray.
I can't remember exactly how many (maybe 4 or 5??), but I know I gave my radiators quite a few thin coats - much better than 1 or 2 thick coats.