Another idea is a couple coats of clear coat spray paint over whatever gold you choose, that should help keep it from becoming dull!

Thanks Shayleigh! I didn't think of that.

Clear shellac; other types of clear coat dissolve the medium, allowing the bronze powder to float back to the top, where it tarnishes. Shellac will not; if you then need weather protection, you can poly on top of the shellac. Use only dexaed shellac to ensure poly-compatibility.
Casey

Where do you dexaed shellac? Never heard of it. When I ask for shellac at HD they want to sell me a can of clear rustoleum spray

This is the stuff (link)
Casey

Here is a link that might be useful: clear de-waxed shellac