I used to deal with this some years back for satellite dishes. Not the ones on your house, the big ones BT has down at Goonhilly and other places.
Those are very expensive to take out of service for repairs, so the protection on the metal struts was;
Heavy galvanising - hot dip, so you need somebody with a swimming pool size bath of molten zinc.
Preparatory wash - can't remember the name, but it sounds like the Mordant wash.
Leave it out of doors for as long as possible.
Etch prime, then 2 coats of 2-part epoxy paint.
I never heard of any deterioration after that had been done.