Primer and paint for radiators
this is the best system to paint anything from plastic to stainless steel

Don't deviate or skip any steps.

1.clean thoroughly with acetone or MEK.

2.mechanically abrade with 180-240 grit (higher for smoother finish, but less lifespan, coarser for rougher finish but better adhesion) sandpaper

3.wipe again with acetone or MEK

4.rust convert any surface rust (on steel only) with 3M marhyde or corroseal rust converter

5.prime with any UMA primer like XIM400

6.paint with devthane 379 aliphatic urethane enamel.

Done!