01-27-2013 02:10 AM - edited 01-28-2013 01:37 AM
Try looking at the webiste for Modern Masters: www.modernmasters.com. This company makes metallic paints which then can have reactive chemicals applied to give that aged copper patina. They claim that they are for exterior use and can be clear coated for protection. Click on Products ->. Metal Effects on the website.
Hope this has helped