You might want to try Dupli-color Clear Coat available at many auto parts stores like AutoZone or Advanced Auto Parts. Comes in a gloss and a matte sheen levels.
It is inteneded to spray, but if you are covering architectural columns I would think you could apply with a brush. If you are going for the rusty industrial chic look, I would think matte would look more natural.
Beware of clear lacquers that can turn amber or even green with UV exposure..
FYI- good clear coats for metals are pricey.
Good luck.

Spray lacquer.

Your choice of sheen, but matte would probably be least visible.

Do not use water based products.
The water in them will cause additional rust resulting in adhesion problems.

We have acquired a very old set of brass fireplace equipment including a fender and coal box all in solid brass (American made). After spending a month polishing, I can see the oxidation beginning. Does anyone know of a business in the Boston, MA area, esp. north shore, that will lacquer these pieces so that I can spend my free time doing something more relaxing?