The Castle Creek fireplace used a 1/8" thick steel plate called laser plate. One could also start with a 'pickled and oiled' plate. A material that is without mill scale. The surface was orbital sanded thru to a 240 grit finish. We used the Birchwood Casey product M20 deluted 50% with distilled water. This patina is a solution that includes selenious and phosphoric acids. The diluted patina was sprayed on with misting bottle and worked in with fine Scotchbrite pads and kept wet with the patina until the desired color was reached. It was then immediately flooded with water and force dried with compressed air and towels. Your metal worker can decide what lacquer to best use, there are many that would work I believe.
http://birchwoodtechnologies.com/antiquing/options.html
ANTIQUE BLACK® M20 IMMERSION
Weggis......looks nice.
Sincerely,
Brent

BRENTON CURTIS METALSMITHING LLC
615 CHEYENNE AVENUE
NEW CASTLE, CO 81647
WWW.BCMETALSMITHING.COM
970.618.3018 c