Depends on your goal. Concourse correct? I believe it's supposed to be a natural galvanized steel finish, with no paint or coating.

Eastwood sells a close duplicate of this look, which will help protect it and give it a uniform coating, but it would not technically be "correct".

I've heard of some clear-coating their bare steel gas tanks in satin clear. Allows the natural finish to show thru for nearly perfect/original look, but also protects it from elements.

I'll let judges comment on whether any of above is a good idea or not for car shows. I'm not into car shows at all, so I don't know what they like...

Many others just paint it silver. I spray-bombed mine with Rustoleum Galvanizing Compound from my local hardware store, but I'm not an originality guy. Looks fine for my driver...but I don't detail my undercarriage, so I'm not looking for perfection.