suggest you get the rust repairs done again but after its all painted(not left in primer as its porous and absorbs moisture= re rusting) then get in behind all the panels and spray fish oil in there, if you cant reach it with a pressure pack then buy a litre can of it from auto one and apply it using a $10 weed sprayer from bunnings
this will enable you to get everywhere that a spray can cant, ive done this with a few cars over the years and it works well