If you're looking for the best method, replace metal with metal, not fibreglass. And no, you don't have to strip the paint to put new paint on. You can just rub it down, prime and paint. Lots of people seem to be under the impression that "strip and paint" is the only way to do it, it isn't, and there are very rarely any advantages of doing it that way. In fact, there are many disadvantages, among them that there is a lot more preperation involved.