Is the paint still bonded to the wood? If so, it would seem that roughing up the paint enough to provide a somewhat fresh and abraded surface would allow new paint to stick. Once you start sanding you can spend lots of money, time, and effort to achieve some form of perfection. Don't ask me how I know.
If the task at hand is to protect the wood underneath with a fresh coat of paint before winter sets in do enough to bond the new paint. Just my two cents, take the advice for what you paid for it!