A quick safety note.. 99% of the paint on those old radiators is lead based paint.. I'd use paint stripper, Lye, or some mechanical means to remove most of the layers of paint, then use a sandblaster for the last cleanup before painting..
I like PPG DP primer/sealer. It is a 2-part epoxy primer that will not come off,, it requires grinding or sandblasting after it drys and sets for more than 8 days.. don't ask how I know that !
If you do your own sandblasting, clean up and bag the left over paint dust.. wetting it down some.. And wear a mask,
I shudder to think about all the lead based paint dust I've inhaled...