Might want to look at this one. I have been considering it for a few years before taking the plunge.
They claim that it's effective for up to 17 years with a touch up around 8-10 years.
It comes as a kit with a 2 part epoxy that you mix with 5 pounds of powdered copper that you apply in 4 coats.
They recommended 3 kits for my 23 so that would be 15 pounds of copper.
Their instructions are good, but haven't met anyone who has used it