No need to remove entire body to bare metal. Tightly adhered paint is great indication of no underlying rust. Having said that, media blasting leaves nothing behind and reveals true nature but is very expensive, albeit.
Dig into the areas you see the rust or paint bubbles. Scrape, sand and wire brush until you get down to bare metal and no loose rust or metal is present.
For example, this fender shows rust and faded paint. The rusty areas need abrasion to bare steel to expose metal failure and repairs needed. The painted areas need a scuff and paint (on a limited budget). One could sand the fenders to bare metal with a few sheets of sandpaper too.
The cab shown needs many hours of metal replacement and can only be repaired properly by a complete body media blast in my opinion... huge job.
Show us some pictures of what you are working on and we will all try to help you.
