I have iron rails on my balcony. Need to use red-oxide primer x2 coats then a suitable metal top coat. Mine have lasted so far 4 years and I live 200 meters from the sea.

 

Just a suggestion:
Aluminium corrodes significantly in a saltwater atmosphere and, as you suggest, eventually carbon steel will rust and show through any coating. Have you thought of using stainless steel? The glass mounting kits in SS can be bought online through various outlets (Carousel, Lazada, Shopee etc). I'm pretty sure the rails etc can be sourced through Polaris as they have the swimming pool rails and stairs fabricated in SS.

 

Thank you. I shall get your advice followed if I cannot source aluminium railings from Cebu.