No need to take it off Graham
we would usually rub it flat and make sure there are no flaky bits , then give it one coat all over.
Good to use the same brand of paint if you can.
A couple of things to be careful of;
1. The sacrificial anodes, fresh water ones are not zinc. Which is the usual material.
2. The water system will be full of fresh water not sea water and the freezing point is much lower. system may need to be drained or run in an antifreeze mix.