The polish itself protects the metal but
it has a limited time that it is effective
depending on exposure. It can last for
years in a controlled environment an
only weeks say on the wheels of a car
that is driven every day. A prop that
is left in the water is a matter of days.
Anything you put on the polished
surface to protect it will take away
some of the shine. (waxing or clear
coating) So to maintain the maximum
shine you polish when it looks like it
needs to be polished. Polished
aluminum requires some maintenance.
Its why people chrome it.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.