Well in marine corrosive its not just water and the steel creating rust. But you have galvinetic corrosive. When you get 2 different types of metal touching and a electrolite you create a battery. Ones a annode and ones a cathode. So it wll still corroded painted or not. You can actually take a volt meter and detect a current flowing. I tested my aluminum rack and my bed. Between then I get .3 volts.