"Unpainted" masts are actually clear anodized. Aluminum, if left bare, will oxidize to a degree and stabilize, but the oxidation leaves a dusty/flaky surface that is unattractive and rubs off on contact.
If the paint is in good shape you should be able to do a scuff sand and repaint. Don't lay it on too thick, as all masts move to some extent and a thick layer of paint will tend to crack over time.
If you do go down to bare metal, then it needs to be thoroughly cleaned after sanding, lightly etched with a mild acidic (we used vinegar) before priming and painting. (We used Awlgrip products)
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)