Craven Morehead writes:

How do I refinish this coping, in place?

Aluminum is about the toughest thing to coat, because stuff won't stick
to oxidized aluminum, which is to say, the surface of any aluminum in
the earthly atmosphere.

You have a big job ahead if you want something that will last.

You must first remove all that old coating and sand the bare metal.

I would then consider a costly polyurethane marine paint system. By
system I mean you have primers specifically for aluminum as well as
finish coats. For example, the Valspar marine polyurethane enamel
(which Home Depot sells, or used to, as it has been recently on
clearance here) starts with a vinyl wash primer with reducer, followed
by vinyl zinc chromate primer in two coats, followed by two to four
coats (!) of the finish enamel.

Polyurethane will stand up to pool chemistry.

Epoxy is another candidate, but that's even more work.

15 years is optimistic for any polymer exposed to pool chemistry and
weather.