It really depends on where the is. Having been in the auto paint & body business a while back, I can tell you one thing for sure. Unless you get rid of ALL of it, it will come back. It's not if, but when. Rust on the surface needs to be sanded or ground away completely to bare metal, any necessary smoothing body work done, then etch primed & painted.
If you're seeing blisters coming from under the paint, unless there is a break in the paint surface where moisture is getting through, it's rusting from the back side. From my experience, (I live in the rust belt, by the way) you're seeing the first sign of rust through which means it's the deepest point of a much larger rusted area that you can't often see. If so, you can try grinding back and filling or welding in a patch, but from my experience, it's like an iceberg. You see the tip, but most of it's hidden.
Panel replacement comes next if the previous steps aren't enough.
You mentioned that you can't get into places, so if you can't sand/grind it all away, or if all the area can't be accessed, coating or saturating tight areas with a rust converter can help especially if you can't afford or don't wish to go nuts with body work. The products I've used are Ospho & Plasti-coat Rust Converter. both leave a primable surface. I've used POR-15 paint on the underside rusted areas with success. On small parts that can be removed, I'd recommend electrolytic rust removal (google it if you're not familiar) then re-painting.