This subject has come up before, you could try a clearcoat with a matting agent added.
As long as you can seal the surface with a waterproof barrier your 'patina' should be preserved.
I would have liked to do the same,but the 'importer' threw about 2 gallons of red oxide primer on mine!

1950 Chevy Advance Design 3100 in Scotland
In the Stovebolt Gallery
More pix on Flickr.
I've definately got this truck thing in my blood ... my DNA sequence has torque settings
"Of all the small nations of this earth,perhaps only the ancient Greeks surpass the Scots in their contribution to mankind" Winston Churchill.