Metal radiator covers — do they have to be painted?

[bungalowrescued]

We have metal radiator covers with decades of paint on them, so we’d like to have them sandblasted. Ideally, we would leave them unpainted. Clear paint, maybe, but no color. The sandblast guy told me they always paint them, and that if we don’t paint them, they’ll rust immediately. Like, within the day. Is this true? There’s no clear paint that can be used??