I had to use conversion coating when I repainted aircraft on the job and I still do on automotive jobs as well. There are various types of brand names available but they are all pretty close to the same formula with the only difference being the type of metal. Aluminum requires it too but the steel formulas are generally a bit different. The phosphoric acid is in all formulas but the amounts of zinc salts and other rust preventative elements are what vary. The basic change is the conversion of any corrosion to a inert form of phosphate and base metal mix.
Etching primers have a very mild form of conversion formula mixed in to the pigment and reducer elements of the primer to promote adhesion.
The conversion formula is similar to rust removers like naval jelly. They will remove corrosion but the don't necessarily have to remove it all. On steel, it just has to convert any remaining corrosion to iron phosphate which is inert. It still has to be primed and painted to keep the dreaded iron oxide at bay.
