Recommendation in treating prior to finishing rust spots. Oshpho is a phosphoric acid rust remover/converter that etches the metal and changes iron oxide to iron phosphate. This is an inert no longer oxidable change of the structure of any remaining rust. It turns the last microspopic particles into iron phosphate and eliminates the return of rust after you have completed the repair. When your down to your last gind and before last light sanding before using whatever you use for cosmetic repair for the problem, use this on the location and it will protect the metal by making any oxidation inert. Then lightly sand as it causes a light sealing surface. Do not use too much as if it gets on plastic or other surfaces it will eat it up. Plastic can melt.
After that the chance of rust returning is nil if done correctly.
Before our new EPA laws it could be found at any auto parts place, now you have to search as many companies had to take it off their shelves like most products that work.
Ospho is also geat for places you cannot repair in tight places where rust is especially the lower skirt of the sprinter in the inside crevices. As long as nothing else is in those locations you can apply liberally as Ospho is like water and will run. In the event of areas you cant reach it will soak into the rust, change the rust to an inert sealer and stopping or significantly slowing return or continuance of rust. Apply with wand in tight spaces or paintbrush, use gloves.
Phosphoric acid works if used with care, but another rust weapon you may want to try is
Evapo-Rust. If you have a Harbor Freight nearby it is sold in-store.
The active ingredient is proprietary and not disclosed but it is allegedly a non-toxic chelating agent that doesn't harm paint. The rust turns into a dark gray powder which you can then lightly rub off (if necessary) with a gloved finger or paper towel, leaving bare steel behind.
The rusted surface needs to be immersed in order for it to work, but I've had good results using it on vertical surfaces by building a little dam out of butyl tape (left over from my roof fan install) and a sheet of plastic, then filling the dam with liquid. It works great for removing the last bits of rust from pitted metal after grinding away the big flakes. There are some good before and after photos in the Amazon reviews.
For $8/qt it's absolutely worth trying before messing around with acid, IMO.