Most of the stock air cleaner housings I've seen were rusty on the inside. I glass bead blast them to get rid of the rust, but that process leads to more rust potential as there is no metal protection left. The next step is obvious, paint it. I use a light coat of flat black on the inside before I paint the outside blue (top and housing), and I don't care if there is a slight amount of blue overspray on the inside (you can't see it anyway). I think the snorkel and heat riser tube were dipped, at least on 65-66 Mustangs.
Jim