A green colour at the water-line is a characteristic of many copper –containing antifoulings and is caused by the formation of “copper patina”. This green patina can also be seen on buildings with copper roofs or copper cladding. A classic example is the Statue of Liberty.
The copper patina consists mainly of "basic copper carbonate", Cu2(OH)2CO3, (BCC) Below the water it is formed most readily in warm water having either an unusually high or low pH, or high oxygen content, or in fresh water. At the waterline it is formed as the copper undergoes atmospheric weathering.
Does the green copper patina affect performance?
Although unsightly, copper patina does not affect antifouling performance since it is very slightly soluble and so continues to releases copper and deter fouling.
How can I prevent “greening” of the waterline?
If you require a specific boottop color around the waterline of your vessel this is best achieved by applying a band of Fiberglass Bottomkote above the antifouling line
Can I overcoat an aged antifouling which has a copper patina?
It is best to remove as much of the copper patina as possible before the paint dries, when the boat is first lifted, using high pressure fresh water washing (HPFWW). Once dried, the patina is much more difficult to remove by HPFWW.
After drying, and provided the patina is strongly adhered and fully intact, it can be over-coated directly with fresh antifouling.
If there is any doubt about the film integrity of the patina and the underlying paint it is recommended that it be lightly wet sanded prior to overcoating.