How do I repair a small leak in a steam radiator? Could this be a big problem?
I haven't had very good luck with repairing steam radiator patches from the outside (sample size: 2). Steam under pressure is very good at finding microscopic holes between patch and substrate, and enlarging them.

What I have done successfully is an inside patch with epoxy, using a wire brush and steel wool to prep the surface. Getting at the leak may be a challenge, particularly with a radiator that has more than 4 sections you may need to break the sections apart (if you do, this is an excellent time to get the exterior sandblasted and painted).

Simplest and most cost-effective solution is probably to find an architectural salvage shop and replace the radiator (they might take your old radiator as a partial trade).