Since no one has addressed the real problem here, recurring rust, I will. Your rust keeps coming back because it is rusting from the inside out, plain and simple. It may not seem like it at first, but from my experience, if and when you clean rust and seal the surface, but the rust keeps coming back, that's because it's rusting from the inside out. You need to inspect the inside of your machine and its internal structure and look for any internal leaks. Once you identify the internal problem and resolve it, then you can again clean the rust and seal the surface because there is no internal rust that will come through again.
I hope this helps.