Hi Geoff
Do you know what kind of blacking was on there originally? I've heard that some kinds don't like to be painted over with different brands - some chemical rejection or something within the two compounds?
Also, that seems quite a short slot for a full blacking. You'd really want to be taking a good amount of time jetwashing all the yuck off the existing blacking before you do anything. Then, a full coat per day really to ensure its fully dried before the next coat. I'd think you'd be best with perhaps 3 coats to give it a good chance of surviving for 3 years approx. Its easy to think you can start adding additional coats while the other is still tacky but it really does need chance to fully dry before the next coat. I'm not sure about your rust question but I'd try and take it back beyond the rust, perhaps to bare metal, score it, prime it and then get some coat of blacking on but I'm far from an expert so its only a personal opinion.
Good luck!
Kay
x