The more boats I see the more convinced I become that corrosion in its myriad forms is wildly variable. So many things change. This year my own boat had lost quite a lot of paint along a 6 inch band at the waterline and below, no idea why, the underneath still had good paint cover, 3 years ago it was the other way round, same paint, same conditions, DIY job both times so quite careful. I think the waterline seems to 'rust' but the base plate tends to pit - I have seen some awful bottoms in my time and also some quite nice ones - sorry, where was I? Ah yes, rust, it seems that the best course of action is to find a good dock or slip that is reasonably affordable and then get the boat out every 3 years and try really hard to maintain the best paint cover possible - I use epoxy but maybe cheaper paint is just as effective with frequent docking?