I'm trying a rust treatment product called "Fluid Film". Recommended by a landscaper, it doesn't dry and claims to stop rust from spreading. It's compatible with auto undercoating and won't harm paint.
Seems like a useful product to halt the cancer when you know you're not going to strip back to metal, treat, paint, etc. - at least not in the near future.
Probably good for those inaccessible areas too.

Wax Oil, Lanolin Wax, Wool Wax, Cosmoline, etc have worked well for years. In the millworks I used Wax Oil in sprayable form to treat machined tool tables that were being put into storage with the addition of a covering of brown industrial waxed paper.