Getting rid of rust is extremely difficult. I've been through it many times. Although that does sound poor.

Bits of mine started to bubble aout a year or so after I had paint done, but a lot of the prep I did myself, and my garage just isn't as dry as you'd like, especialyl in winter. So although it was "dry", and I did epoxy priper over the repairs....it still just wasn't clean and dry enough in some cases.

And most rust killers....you end up rubbing them off anyway to get smooth enough for primer/paint.

Only true way to deal with rust, is cut it all out, which is a huge task too. All mega expensive labour intensive work. I spent many many hours, and I know some corners were cut. It really is specialist work.