What is a smoker box? If that is the little thing with louvers which is loaded with chips an placed in the gasser? I would just go buy another or somebody be glad to mail you one for the shipping and handling charges. Now if we speaking of a firebox on a manly man type bbq pit..we need more info like where is the rust starting and finishing etc. Hopefully it aint afflicting the insulated firebox. Give more scoop thanks. What is up with cooking all that stuff in the foil pans? I own two and have used one daily for five years at least. Get a grip here.
I have never seen any paint that will stand up to a firebox temps over time.
I usually have some spray cooking oil around and I spray the outside down when it is hot. And again when it cools. The oil will harden like varnish (season a cast iron pan) when it gets hot. You can wipe on some rendered fat from inside with the same results.
AND NEVER LET THE ASHES IN THE FIREBOX GET WET! They will corrode that side without fail. Same fix: mix rendered fat from the cooking side with the ashes and scoop out the mixture and wipe down the inside to leave a thin coat of grease, and the inside will not get wet as easily.